Leslie was a 23rd century Starfleet officer. He served aboard the USS Enterprise between 2265 and 2268 under the command of Captain James T. Kirk, during the historic five-year mission. By 2266, he held the rank of lieutenant.
Leslie was a versatile crew member who was cross-trained to serve in several departments as needed. He was assigned primarily to the operations division and was usually posted to the engineering station on the bridge of the Enterprise, next to the turbolift doors. He also performed many tasks as a security officer.
He is one of the few officers to have worn a blue, red, and gold Starfleet uniform. He also wore an earlier-style beige operations division uniform. He performed some duties in a red or blue jumpsuit. After stardate 2126.1, his uniform regularly showed the rank insignia of a lieutenant. (TOS: "Where No Man Has Gone Before", "The Changeling", "Assignment: Earth", "The Enemy Within", "The Squire of Gothos")
His service began on stardate 1312.4, with his first assignment occurring on stardate 1313.1. (TOS: "Where No Man Has Gone Before") During breaks from duty, he socialized with his shipmates in the recreation room of the Enterprise or in a bar. (TOS: "Charlie X", "The Trouble with Tribbles"; DS9: "Trials and Tribble-ations")
He risked facing a charge of mutiny on stardate 3417.3. After becoming infected by the Omicron spores, he abandoned his post and lined up outside the transporter room to join the colonists of Omicron Ceti III. He openly disobeyed Kirk's order when told to return to his station. He continued his service after the spores had lost their effect on him. (TOS: "This Side of Paradise")
During a planetary survey of Argus X on stardate 3619.2, he was killed by the dikironium cloud creature after it had drained all the red blood corpuscles from his body. He was revived by stardate 3620.7 and soon returned to active duty. (TOS: "Obsession") His service aboard the Enterprise continued until his last assignment on stardate 5630.7. (TOS: "Is There in Truth No Beauty?")
Leslie was a duty officer on two occasions. He first sat in the command chair on stardate 2126.1, when Kirk and several other officers escaped safely from Trelane of the planet Gothos. (TOS: "The Squire of Gothos") The next 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 Alternative Factor")
Leslie performed many engineering tasks during his time aboard the Enterprise. He was at the engineering station during the tense standoff with the Fesarius on stardate 1512.2 and the battle with a Romulan Bird-of-Prey on stardate 1709.1. (TOS: "The Corbomite Maneuver", "Balance of Terror") He was at the same post when the Enterprise crew first encountered Harcourt Fenton Mudd, learned the fate of Doctor Roger Korby, and visited Miri's homeworld. (TOS: "Mudd's Women", "What Are Little Girls Made Of?", "Miri")
He was on the bridge when Doctor Simon Van Gelder burst in with a phaser. (TOS: "Dagger of the Mind") He was unable to stop a cordrazine-crazed Doctor Leonard McCoy from escaping the bridge and transporting down to the time vortex planet. (TOS: "The City on the Edge of Forever")
He passed out after Khan Noonien Singh cut off the life support system to the bridge and hijacked the Enterprise on stardate 3141.9. He was revived later and temporarily taken prisoner by Khan's followers. (TOS: "Space Seed") He also fainted on the bridge and was quickly treated by a medical team during the starship's journey through the space amoeba. (TOS: "The Immunity Syndrome")
He assisted Lieutenant Galloway in the transporter room and beamed in the Galileo survivors before the shuttlecraft incinerated around Taurus II. (TOS: "The Galileo Seven") He was attacked in the same room by an infected Commander Spock on stardate 3287.2 when the first officer applied the Vulcan nerve pinch on him in an attempt to beam down to the planet Deneva. (TOS: "Operation -- Annihilate!") He was also rendered unconscious by Gary Seven during the time traveling mission to 1968. (TOS: "Assignment: Earth")
When the Enterprise accidentally hurled back in time to the year 1969, he repaired the engineering station for the voyage back to 2267. He assisted Spock and Lieutenant Hikaru Sulu in finding a weakness in Apollo's force field. He also aided Lieutenant Commander Montgomery Scott in weakening Vaal's hold on the starship. (TOS: "Tomorrow is Yesterday", "Who Mourns for Adonais?", "The Apple")
He was the bridge engineer when Commodore Stocker assumed command of the Enterprise on stardate 3479.4 and ordered the vessel to enter the Romulan Neutral Zone. (TOS: "The Deadly Years") He performed the same duty when Commodore Matt Decker took command on stardate 4202.9 and used the Enterprise to attack the "planet killer." (TOS: "The Doomsday Machine")
He was on the bridge after a group of Kelvans hijacked the ship and temporarily neutralized and reduced him into a small dehydrated porous cuboctahedron solid. (TOS: "By Any Other Name") He was stunned into unconscious when the Eymorg Kara appeared and removed Spock's brain. (TOS: "Spock's Brain")
While the Enterprise was under attack by a Klingon battle cruiser on stardate 4372.5, he gave up his seat at the engineering station to Elaan, the Dohlman of Elas. He continued his duties while standing until he was relieved by Scott. (TOS: "Elaan of Troyius")
Leslie was first assigned to the helm on stardate 1513.1, during the mission to planet M-113. He took over the helm when an infected Sulu abandoned his post on stardate 1704.2. He was later relieved of duty after he succumbed to the effects of the Psi 2000 virus. (TOS: "The Man Trap", "The Naked Time")
He was at the helm when the Enterprise discovered the rogue planet Gothos, transported the Karidian Company of Players to Benecia, orbited Pyris VII when Lieutenant DeSalle was in command, and visited the Shore Leave Planet. (TOS: "The Squire of Gothos", "The Conscience of the King", "Catspaw", "Shore Leave")
He performed tactical duties on stardate 3088.7. Under Kirk's orders, he fired the starship's phasers and destroyed Lazarus' spaceship. This sealed the negative magnetic corridor and saved the two universes from mutual destruction. (TOS: "The Alternative Factor")
Leslie treated Spock twice as a medical assistant in sickbay. The first time happened when the first officer was attacked by the M-113 creature on stardate 1513.8. (TOS: "The Man Trap") The next one was on stardate 3372.7, when Spock underwent a medical examination by McCoy which revealed that he was suffering from the effects of pon farr. (TOS: "Amok Time")
Leslie was the navigator on stardate 3541.9, when the Enterprise encountered the intersteller space probe Nomad. He was one of two officers to carry Scott's body to sickbay after the space probe attacked the chief engineer on the bridge. He was not present to witness Scott's revival. (TOS: "The Changeling")
Leslie was a science officer on the Alfa 177 geological survey team on stardate 1672.1. He suffered frostbite and recovered from the ordeal. (TOS: "The Enemy Within") He was posted to the science station on stardate 3211.7, as the Enterprise headed for Triskelion in search of Kirk, Lieutenant Nyota Uhura and Ensign Pavel Chekov. (TOS: "The Gamesters of Triskelion")
Leslie's first assignment aboard the Enterprise happened on stardate 1313.1. He guarded the holding cell containing Lieutenant Commander Gary Mitchell at the Delta Vega Station. He was stunned into unconsciousness and later recovered after Mitchell unleashed his psionic powers on him to escape. (TOS: "Where No Man Has Gone Before") He was a member of the Beta III landing party on stardate 3156.2 and was temporarily absorbed into the Body of Landru. (TOS: "The Return of the Archons") He joined Lieutenant Commander Giotto's security team in the hunt for the Horta on stardate 3196.1. He was clubbed on the head three times by an angry mob of Janus VI miners and later healed of his injuries. (TOS: "The Devil in the Dark")
He was assigned to the security team aboard Deep Space Station K-7 on stardate 4523.3. 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")
He was part of the security detail for the Arret landing party on stardate 4768.3. When Kirk, Spock, McCoy and Doctor Ann Mulhall beamed over to the planet, Sargon prevented him from joining along. (TOS: "Return to Tomorrow") Later that year, he held custody of Ronald Tracey after the captain of the USS Exeter was charged by Kirk for violating the Prime Directive on Omega IV. He was exposed to the Omega IV virus but did not suffer from its effects. (TOS: "The Omega Glory") He shadowed Tommy Starnes after the Enterprise rescued the orphans of the Starnes Exploration Party on stardate 5029.5. He was posted by the turbolift doors when the orphans hijacked the starship and was unable to comply with Kirk's orders after Starnes changed the captain's words into incomprehensible gibberish. (TOS: "And the Children Shall Lead")
His last assignment aboard the Enterprise occurred on stardate 5630.7. After Kirk discovered an assassination plot against Ambassador Kollos, Leslie teamed up with the captain, Spock, McCoy and Ensign Roger Lemli in response to the threat and ensured that no harm came to the Medusan ambassador. (TOS: "Is There in Truth No Beauty?")
Leslie was part of an elite group of Enterprise officers who dined at a banquet with Khan on stardate 3141.9. He was on the bridge when the Augment leader cut off the life support system. A weakened Kirk listed the bridge crew for commendations before passing out. Leslie was not included on the list. (TOS: "Space Seed")
On stardate 4729.4, Leslie was one of twenty officers selected by the M-5 computer to remain aboard the Enterprise for the war games exercise. He was one of two security guards to escort Doctor Richard Daystrom to sickbay when the renowned scientist suffered a nervous breakdown. (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 was never addressed by his Starfleet service rank. Kirk simply called him "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 the comic book Who's Who in Star Trek, Leslie grew up on Earth near the Starfleet Impulse Engine Testing Grounds in El Paso, Texas, where he developed an interest in engineering. He went to Starfleet Academy in San Francisco and met Kirk in judo competitions. In the novel Present Tense, he developed a friendship with Sulu on the Enterprise soon after the events of "The Naked Time".
The novel Yesterday's Son mentions 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 novel 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. The novel The Fire and the Rose mentions that he attended Kirk's memorial service at Starfleet Academy in 2293 during the events of Star Trek Generations.
There are inconsistencies with regards to Leslie's first name. According to Star Trek Short Stories and Who's Who in Star Trek, his first name is Frank. In the novels The Fire and the Rose and A Choice of Catastrophes, it is Ryan. The other novels state that it is Ed – an apparent tribute to Eddie Paskey, the actor who portrayed him on the original series.