Dr. Thomas "Tom" Leighton was a male Human civilian in the 23rd century. He was an empirical research scientist; Spock, who knew of him, described him to Kirk as, "Steady, reputable, occasionally brilliant". In 2266, he lived with his wife Martha Leighton on Planet Q.
In 2246, he was living on Tarsus IV when four thousand people were executed on the orders of Governor Kodos. Nine eyewitnesses, including Leighton, could identify the governor on sight. Leighton and Kirk appeared to have been personal acquaintances, perhaps as a result of their shared experiences on Tarsus IV. He was tormented by what happened that day for the next twenty years.
In 2266, he diverted the USS Enterprise to Planet Q, ostensibly to confirm his discovery of a new synthetic food that could totally end the threat of famine on Cygnia Minor, a nearby colony. In fact, he diverted the Enterprise because he believed Anton Karidian, a traveling performer, was actually Kodos. He hoped Kirk, another of the eyewitnesses, would help him in his quest to bring Kodos to justice. Kirk wasn't convinced until, ironically, Leighton was found dead. Lenore Karidian, insanely desperate to protect her father from his past, had murdered Leighton; he was the seventh victim of her quest to eliminate all those who could identify Kodos. He was survived by his wife Martha. (TOS: "The Conscience of the King")