Thomas Barrow was the first footman in Downton Abbey before the war, later enlisting as a medic in an attempt to save himself from the frontlines. He is good friends with O'Brien and the two of them are constantly plotting to get John Bates thrown out, none of which work. He is also a thief and steals some of the Abbey's Chateaux Rouge wine. John Bates sees him so Thomas turns the tables on him and makes out to Mr Carson that Bates took it. Bates then does a bit of table-turning himself and reveals that he has not done it. When the hint of a war approaches, Thomas says he does not want to go to battle and signs up to be a doctor. He is gay, and has an affair with the Duke of Crowborough. He tries to get in with Kemal Pamuk, the Turkish envoy, but Pamuk slaps his hand away and says he will tell Lord Grantham if Thomas does not tell him where Mary's room is. Thomas is one of the few people who knew that Kemal was in Mary's room the night he died. While in the trenches, alongside Matthew, he holds a lighter above the trench wall to get his hand shot, and effectively be removed from service. Through this action he gets sent back to Downton Abbey. He had feelings for Lieutenant Edward Courtenay while he was recovering from gas-blindness. This is one of the first times Thomas is seen with a very kind and caring side. After being told that he was to be moved out of the hospital to make room for newer, terminal patients Edward committed suicide with a razor he sneaked into his bed. Thomas is seen crying and is left alone again. After the war Thomas tried to make it as a black market buisnessman but after buying goods that were fake and spending all of his money, Thomas was forced to return to Downton abbey as a footman.
Thomas has shown himself to be capable of being very cruel, for example, he asked Daisy to go to the fair with him just to upset William who was planning to ask her. Also he later made comments about William's mother after she had died, which resulted in a fight. However, as the series progressed Thomas appeared to learn some humanity and his attitude toward working as a footman became much more positive. Fans speculate that Thomas' failed relationships (e.g. with the Duke) made him bitter.
Thomas tells O'Brien that his father was a clockmaker.