In 2246, he was living on Tarsus IV when four thousand people were executed on the orders of Governor Kodos. Leighton was one of nine eyewitnesses who could identify the governor on sight. Leighton and James T. 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.
By the 2260s, Leighton worked as an empirical research scientist, who according to Spock, was described as "steady, reputable, occasionally brilliant." In 2266, he lived with his wife, Martha Leighton, on Planet Q.
In 2266, Leighton 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 that 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. Leighton was survived by his wife Martha. (TOS: "The Conscience of the King")
Thomas Leighton was portrayed by William Sargent.
In the final revised draft script of "The Conscience of the King", Dr. Leighton was initially described merely as "a contemporary of Kirk's."
Although it was not explicitly stated, conversational context strongly suggested that the left side of Leighton's face was ruined during the massacre at Tarsus IV. The script didn't specify this injury, merely referring to Leighton having a "crippled body" instead. In a deleted scene from "The Conscience of the King", Leighton was established as having lost his parents and two brothers during the massacre, and having experienced, ever since then, nightmares about the incident. ("Swept Up: Snippets from the Cutting Room Floor", Star Trek: The Original Series - The Roddenberry Vault special features) Later in the script, upon Kirk breaking the news to Spock that there hadn't actually been any synthetic food samples and Spock replying that would need to be noted due to the seriousness of diverting a starship, Kirk remarked, "Well, a black mark against Dr. Leighton isn't going to hurt him now."
Who's Who in Star Trek 1 mistakenly calls him Robert Leighton.
Leighton appears as a supporting character in the 2018 Star Trek: Discovery novel Drastic Measures, which positions him and his parents as friends of Jim and Winona Kirk in the Tarsus IV capital, New Anchorage. It's revealed that Leighton's disfigurement came from suffering phaser burns when Governor Kodos ordered the execution of 4,000 colonists in the city's amphitheater, and being in proximity to phasers set to disintegrate.
Leighton watched in horror as his parents were murdered, and when a Starfleet relief ship arrived at the planet, Leighton was treated for his injuries, and was virtually catatonic. While convalescing, Jim made an effort to find a photograph of Kodos in the colony's computer systems, and soon thereafter, both Kirk and Leighton met Commander Philippa Georgiou, who was engaged in a manhunt for Kodos on the planet's surface.