Even though he was primarily assigned to the operations division, Lieutenant Leslie was a versatile officer who was cross-trained to serve in several departments as needed. In most of his duties on the Enterprise, he worked at the engineering station on the bridge, just next to the turbolift doors.
The five-year mission
Under the command of Captain James T. Kirk, Leslie performed a variety of functions on the Enterprise, and the different Starfleet uniforms he wore reflected it. At the start of the five-year mission, he wore both a beige operations division uniform and a blue sciences division uniform. After this style was replaced by the newer one in 2266, he regularly wore the red operations division uniform whenever he performed the duties of security officer or engineer. (TOS: "Where No Man Has Gone Before")
Occasionally, he sported the gold command division uniform whenever he was posted as the ship's navigator. He sometimes wore this uniform as he walked along the corridors of the Enterprise. (TOS: "Assignment: Earth", "The Menagerie, Part I", "Obsession", "The Deadly Years", "Return to Tomorrow", and more)
He also sported two different duty jumpsuits – a red one if he was assigned to main engineering, and a blue one whenever he worked in sickbay or was assigned to the sciences division of a landing party. (TOS: "Assignment: Earth", "The Man Trap", "Amok Time", "The Enemy Within")
Before stardate 2126.1, his uniform showed no rank insignia until he was promoted to the rank of lieutenant. With rare exceptions after this point, his uniform showed a full lieutenant stripe. (TOS: "The Squire of Gothos")
The first time happened after he became a lieutenant on stardate 2126.1. He was the duty officer when Kirk and several other Enterprise officers safely escaped from Trelane of the planet Gothos. The second time happened on stardate 3088.7, when the starship investigated the cause of massive spatial disturbances on an iron-silica planet and encountered Lazarus. (TOS: "The Squire of Gothos", "The Alternative Factor")
Of all the duties he had on the Enterprise, Leslie was well known for his skills as an engineer. Even though he was occasionally assigned to the transporter room and main engineering, he was regularly posted as a bridge engineer on many missions.
He was at the engineering station when the Enterprise crew first encountered Harcourt Fenton Mudd, learned the fate of Doctor Roger Korby, and visited Miri's homeworld. Later that year, he witnessed Doctor Simon Van Gelder burst onto the bridge with a phaser. (TOS: "Mudd's Women", "What Are Little Girls Made Of?", "Miri", "Dagger of the Mind")
The following year, he was at the same post when the Enterprise returned to Talos IV, pursued the Gorn ship, and orbited the Halkan homeworld. As the bridge engineer, he also was unable to stop a cordrazine-crazed Doctor Leonard McCoy from escaping the bridge. (TOS: "The Menagerie, Part I", "The Menagerie, Part II", "Arena", "Mirror, Mirror", "The City on the Edge of Forever")
His engineering skills were put to the test in four missions. When the Enterprise accidentally hurled back in time to the year 1969, he helped repair the engineering systems for the voyage back to 2267. On stardate 3468.1, he helped Commander Spock and Lieutenant Hikaru Sulu in breaking the Enterprise free from Apollo's force field. On stardate 3614.9, he tried to restore the engineering station after Redjac took over the ship's computer systems. He also helped repair the engineering systems on stardate 3715.3, when Lieutenant Commander Montgomery Scott tried to break the starship free from Vaal's hold. (TOS: "Tomorrow is Yesterday", "Who Mourns for Adonais?", "Wolf in the Fold", "The Apple")
Also noteworthy is the fact that he was the bridge engineer when two Starfleet commodores assumed command of the Enterprise. The first time happened on stardate 3479.4, when Commodore Stocker ordered the ship to enter the Romulan Neutral Zone. The second time occurred on stardate 4202.9, when Commodore Matt Decker used the Enterprise to attack the "planet killer." (TOS: "The Deadly Years", "The Doomsday Machine")
Perhaps the only time that he came into contact with a royal dignitary while posted as a bridge engineer was when the Enterprise was under attack by a Klingon battle cruiser on stardate 4372.5. After Elaan, the Dohlman of Elas, entered the bridge, he offered his seat to her and performed his engineering duties while standing. He continued this until he was relieved by Scott. (TOS: "Elaan of Troyius")
Before he received his promotion, Leslie performed the duties of a helm officer on three missions. The first time occurred on stardate 1513.1, when the Enterprise orbited planet M-113. He took over the helm again on stardate 1704.2, after an infected Sulu abandoned his post. The third time happened on stardate 2124.5, when the ship discovered the rogue planet Gothos. (TOS: "The Man Trap", "The Naked Time", "The Squire of Gothos")
After his promotion, he operated the helm four more times. The first one occurred on stardate 2817.6, when the Enterprise transported the Karidian Company of Players from Planet Q to Benecia. The next one happened on stardate 3018.2, when Lieutenant DeSalle was placed in command of the Enterprise during the mission to Pyris VII. The third one happened on the visit to the Shore Leave Planet on stardate 3025.3. (TOS: "The Conscience of the King", "Catspaw", "Shore Leave")
His most noteworthy assignment at the helm occurred on stardate 3088.7, when he performed some tactical duties. Under Kirk's orders, he fired the starship's phasers and destroyed Lazarus' spaceship. Not only did this action seal the negative magnetic corridor, it also saved two universes from mutual destruction. (TOS: "The Alternative Factor")
At times, Leslie helped Doctor McCoy in sickbay as a medical assistant. On two different occasions, he treated Spock. The first one occurred on stardate 1513.8, after the first officer was attacked and injured by the M-113 creature. The second one happened on stardate 3372.7, when Spock underwent a medical examination on Kirk's orders – an examination which revealed that the first officer was suffering from the effects of pon farr. (TOS: "The Man Trap", "Amok Time")
There is one occasion when Leslie performed the duties of a navigator. This happened on stardate 3541.9.
When the Enterprise was en route to the Malurian star system to investigate a distress call, he was at the navigation console as the starship encountered the interstellar space probe Nomad. After Nomad attacked Scott on the bridge, he was one of the two officers who helped carry the chief engineer to sickbay. (TOS: "The Changeling")
Leslie served as a science officer on two different missions. Before he became a lieutenant, he joined the Alfa 177 geological survey team on stardate 1672.1, and survived the planet's harsh environment. As a lieutenant, he operated the Enterprise science station on stardate 3211.7 as the ship headed for Triskelion in search of Kirk, Lieutenant Nyota Uhura and Ensign Pavel Chekov. (TOS: "The Enemy Within", "The Gamesters of Triskelion")
In addition to his engineering accomplishments, Leslie was well known for his skills as a security officer.
Early in his career, he guarded the Delta Vega holding cell containing Lieutenant Commander Gary Mitchell on stardate 1313.1. The following year, he was part of the security detail for a Beta III landing party. On Janus VI, he joined Lieutenant Commander Giotto's security team in the hunt for the Horta. (TOS: "Where No Man Has Gone Before", "The Return of the Archons", "The Devil in the Dark")
His security duties increased on stardate 4523.3, when he was assigned to the security team aboard Deep Space Station K-7. While posted there, he helped end a bar brawl between the Klingons and the Enterprise crew. He also guarded the quadrotriticale storage compartment, apprehended Cyrano Jones for questioning, and assisted Kirk in uncovering the true identity of Arne Darvin. (TOS: "The Trouble with Tribbles"; DS9: "Trials and Tribble-ations")
After the mission on K-7, he had several other notable assignments. On stardate 4768.3, he was part of the security detail for the Arret landing party. When Kirk, Spock, McCoy and Doctor Ann Mulhall beamed to the planet, Sargon prevented him from joining along. (TOS: "Return to Tomorrow")
Later that year, he joined Sulu and beamed down to a Kohm village on Omega IV. He held custody of Captain Ronald Tracey after Kirk charged the captain of the USS Exeter for violating the Prime Directive. (TOS: "The Omega Glory")
On stardate 5029.5, he was assigned to watch over Tommy Starnes when the Enterprise rescued the orphans of the Starnes Exploration Party. Later, he was posted by the turbolift doors after Starnes and the other children took control of the starship. When Kirk tried to regain control, he was unable to follow the captain's orders after Starnes transformed Kirk's words into incomprehensible gibberish. (TOS: "And the Children Shall Lead")
His final assignment occurred on stardate 5630.7, when Kirk discovered an assassination plot against Ambassador Kollos. He teamed up with Kirk, Spock and McCoy in response to the threat and ensured that no harm came to the Medusan ambassador. (TOS: "Is There in Truth No Beauty?")
Health and vitality
As a Starfleet officer, Leslie was exposed to many dangers. Some were diseases from alien viruses. Others were physical assaults. In a few cases, he had a few brushes with death. Fortunately, he was healthy enough to survive all these ordeals until his service on the Enterprise ended.
Like any Enterprise officer, Leslie suffered illnesses. Among the most notable, five stand out – one which was nearly fatal.
During the mission to Alfa 177 on stardate 1672.1, he nearly froze to death after suffering from frostbite and hypothermia. Fortunately, he survived this ordeal and returned to active duty by stardate 1704.2 – just before he was infected with the Psi 2000 virus on stardate 1704.3 being carted from the helm (where he had gone to replace an infected Sulu) and off the bridge. (TOS: "The Enemy Within", "The Naked Time")
On stardate 3417.3, he was exposed to the Omicron spores. After being infected, he abandoned his post and lined up outside the transporter room. When Kirk ordered the crew to head back to their stations, he openly defied the captain and refused to obey the order. The spores lost their influence on him after Kirk and Spock built a subsonic transmitter. (TOS: "This Side of Paradise")
After exposure to the negative energy field of a space amoeba on stardate 4307.1, he collapsed on the bridge. Fortunately, a medical team was on hand to quickly treat him. Later that year, he was infected with the Omega IV virus. However, he spent enough time on Omega IV that he built up sufficient immunity and naturally recovered. (TOS: "The Immunity Syndrome", "The Omega Glory")
Assaults and injuries
His first assault happened at the Delta Vega lithium cracking station on stardate 1313.3. He was rendered unconscious after a psionic Gary Mitchell unleashed his powers on him. The only other time a Starfleet officer attacked him was on stardate 3287.2. In this incident, an infected Spock applied the Vulcan nerve pinch on him as the Enterprise orbited Deneva. (TOS: "Where No Man Has Gone Before", "Operation -- Annihilate!")
Before the nerve pinch attack, he was a victim of Khan Noonien Singh earlier that year. He passed out after Khan cut off the life support system to the bridge on stardate 3141.9. He was later revived and taken prisoner by Khan's followers. (TOS: "Space Seed")
Later that year, he suffered head trauma. This occurred on stardate 3196.1, when the Janus VI miners clubbed him. Fortunately, he recovered from his head injuries in time to resume his duties for the Federation-Klingon War of 2267. (TOS: "The Devil in the Dark", "Errand of Mercy")
Of all the assaults, none were more deadly to him than the attack of the dikironium cloud creature on stardate 3619.2. He was declared dead on Argus X after the creature drained all the red blood corpuscles from his body. However, by stardate 3620.7, he recovered and began walking the corridors of the Enterprise. By stardate 3715.3, he returned to active duty. (TOS: "Obsession")
After the attack on Argus X, he experienced four more assaults. The first one came from Gary Seven who used his servo to render him unconscious. The next two happened on stardate 4657.5. When the Kelvan Milky Way Expedition attempted to hijack the Enterprise, to return to their home world in the Andromeda Galaxy, Leslie was on the bridge when the Kelvan Hanar transported himself onto the bridge and put Leslie and the rest of the bridge crew into temporary 'frozen' stasis. On stardate 4658.9, the Kelvan Drea neutralized him into a dehydrated cuboctahedral crystalline solid block, the size of a human fist and that represented the "distilled" essence of his being, because they considered Leslie to be one of many of the non-essential personnel. Leslie was restored to normal after Kirk, Spock, McCoy and Scotty, the only four members of the crew not neutralized, regained control of the Enterprise. The last one occurred on stardate 5431.4, when Kara used her control bracelet to render him unconscious. He was not harmed in these four incidents. (TOS: "Assignment: Earth", "By Any Other Name", "Spock's Brain")
During his service aboard the Enterprise, Leslie received two noteworthy recognitions. The first one happened on stardate 3141.9, when he was part of an elite group of officers who dined with Khan. After the Augment leader cut off life support to the bridge, an apparently disoriented Kirk listed the bridge crew for commendations before passing out – a list which excluded Leslie despite the fact that he was there at the time. (TOS: "Space Seed")
His second recognition occurred on stardate 4729.4, when the M-5 computer took control of the Enterprise and selected him as one of the twenty officers for the war games exercise. After Doctor Richard Daystrom suffered a nervous breakdown, he was one of two security guards who escorted the renowned scientist to sickbay. (TOS: "The Ultimate Computer")
- In "The Conscience of the King", Leslie sits at the helm and awaits Kirk's orders to depart from Benecia.
- Kirk: "Whenever you're ready, Mister Leslie."
- Leslie: "Leaving orbit, sir."
- Kirk: "Get back to your stations."
- Leslie: "I'm sorry, sir. We're all transporting down to join the colonists."
- Kirk: "I said get back to your station."
- Leslie: "No, sir."
- Kirk: "This is mutiny, mister."
- Leslie: "Yes, sir. It is."
- "Where No Man Has Gone Before" (TOS Season 1)
- "The Corbomite Maneuver"
- "Mudd's Women"
- "The Enemy Within"
- "The Man Trap"
- "The Naked Time"
- "Charlie X"
- "Balance of Terror"
- "What Are Little Girls Made Of?"
- "Dagger of the Mind"
- "The Conscience of the King"
- "The Galileo Seven"
- "The Menagerie, Part I"
- "The Menagerie, Part II"
- "Shore Leave"
- "The Squire of Gothos"
- "The Alternative Factor"
- "Tomorrow is Yesterday"
- "The Return of the Archons"
- "Space Seed"
- "This Side of Paradise"
- "The Devil in the Dark"
- "Errand of Mercy"
- "The City on the Edge of Forever"
- "Operation -- Annihilate!"
- "Catspaw" (TOS Season 2)
- "Friday's Child"
- "Who Mourns for Adonais?"
- "Wolf in the Fold"
- "Amok Time"
- "The Doomsday Machine"
- "The Changeling"
- "The Apple"
- "Mirror, Mirror"
- "The Deadly Years"
- "The Trouble with Tribbles"
- "Bread and Circuses"
- "Journey to Babel"
- "A Private Little War"
- "The Gamesters of Triskelion"
- "The Immunity Syndrome"
- "A Piece of the Action"
- "By Any Other Name"
- "Return to Tomorrow"
- "The Ultimate Computer"
- "The Omega Glory"
- "Assignment: Earth"
- "Spectre of the Gun" (TOS Season 3)
- "Elaan of Troyius"
- "The Enterprise Incident"
- "And the Children Shall Lead"
- "Spock's Brain"
- "Is There in Truth No Beauty?"
- DS9: "Trials and Tribble-ations"
Leslie was played by Eddie Paskey, a regular extra who served as a set stand-in throughout the course of the original Star Trek series. He was often seen in the background wearing a red Starfleet duty uniform while operating the engineering station on the Enterprise bridge or serving as a security officer.
Paskey had few speaking opportunities throughout the series. The first time he spoke occurred in "The Naked Time", when his character replied to Kirk's orders on the bridge. The other times he was heard speaking occurred in "The Conscience of the King", "The Alternative Factor", and "This Side of Paradise".
Leslie's last filmed appearance happened on the episode "Is There in Truth No Beauty?", when Paskey retired from the series. The actor suffered a back injury while filming a scene in "Operation -- Annihilate!", where Spock applied the Vulcan nerve pinch on Leslie. He also experienced severe headaches which were aggravated by the stage lighting on the set. By the beginning of the third season, Paskey felt that he could no longer handle the job's activities. 
Paskey's character did not have a set identity at the start of the series. The script for the pilot episode, "Where No Man Has Gone Before", referred to him as both a "crewman" and a "guard". He was once called "Connors" by McCoy in "Mudd's Women". By the end of "The Conscience of the King", Kirk addressed him as "Mister Leslie" three times, and continued calling him "Leslie" in subsequent episodes.
Leslie has the distinction of being known as the first Star Trek recurring character to return from the dead. This is evident in "Obsession", when he was killed by the cloud creature and listed by Kirk as one of the two unnamed and deceased officers in his log entry. Later in the same episode, he inexplicably appears twice – walking by Kirk's quarters and walking past sickbay.
According to Paskey, a scene in the script for "Obsession" that had all three victims of the landing party attacked by the creature would have them coming back to life later in sickbay due to a miracle potion. He mentioned that this scene was never filmed.  Director Ralph Senensky confirmed that he did not film it. 
Leslie's Starfleet service rank was never explicitly mentioned throughout the five-year mission, especially since Kirk often addressed him as "Mister Leslie". Until he sat in the captain's chair in "The Squire of Gothos", Leslie's uniform did not display any rank insignia. With three exceptions after this point, his uniform regularly showed a full lieutenant stripe.
When he was attacked by the cloud creature in "Obsession", the rank stripe was missing. It was also absent in "Spock's Brain" and "Is There in Truth No Beauty?", Paskey's last two filmed episodes on the series.
According to Who's Who in Star Trek, he grew up on Earth near the Starfleet Impulse Engine Testing Grounds in El Paso, Texas, where he presumably developed an interest in engineering. In Present Tense, he developed a friendship with Sulu soon after the events of "The Naked Time".
With respect to his career in Starfleet, Yesterday's Son shows that he achieved the rank of lieutenant commander and served as the chief engineer aboard the USS Lexington near the end of Kirk's five-year mission. Shortly after the events of Star Trek: The Motion Picture, as depicted in The Darkness Drops Again, he was transferred back to the Enterprise and served as the security chief under Captain Spock. In Who's Who in Star Trek, he returned to the USS Lexington around the time of Star Trek IV: The Voyage Home.
With regards to his first name, there have been some inconsistencies. According to Star Trek Short Stories and Who's Who in Star Trek, Frank is his first name. In Present Tense, it is Ed – an apparent homage to Eddie Paskey, the actor who portrayed him on the Original Series. However, in The Fire and the Rose, it is Ryan – an apparent reference to the name speculation found in "The Naked Time".