Thomas Barrow is underbutler at Downton Abbey having formerly worked there as both first footman and head valet as well as serving in the army as a Corporal and later Lance Sergeant. He arrived at Downton in 1910, indicating in 1920 he had worked there for ten years.

Little is known of Thomas's family but he does tell Sarah O'Brien and later Jimmy Kent that his father was a clockmaker. As a result, he apparently has a natural affinity with clocks, saying that he grew up with them and "understands" them. O'Brien calls him the house's clock expert. He has worked in Downton for over 10 years and as a footman used to wind all the clocks in the house. He also appears to be quite talented at cricket, Lord Grantham stating as much, noting he got most of the house team's runs in the 1919 match. He mentions having a cousin in Bombay to Bates.
He is homosexual, and once had an affair with the Duke of Crowborough, while the Crawleys were in London.