Thomas was general secretary during the successful national rail strike of 1919 that was jointly called by the NUR and Associated Society of Locomotive Engineers and Firemen against proposed wage reductions.

He was re-elected in the 1918 general election and was considered as a potential candidate for the role of Chairman of the Parliamentary Labour Party and by extension Leader of the Opposition.

He was accused of leaking Budget secrets to his stockbroker son, Leslie Thomas, and Alfred Cosher Bates, a wealthy businessman.

In a Judicial Tribunal set up by the government, Bates admitted giving Thomas £15,000 but claimed it was an advance for a proposed autobiography.

This high sum for an autobiography, not yet written, only increased suspicion of the two men's relationship, and Thomas was forced to resign from the government and House of Commons.

Among the Labour ministers he was a favourite with George V.[1] It was from laughing at a bawdy joke Thomas told the king that the latter split a post-operative wound from lung abscess surgery, delaying his recovery to near the 1929 General Election.

Thomas's custom of wearing a dress suit is cited as an apparent certainty that could fail unlike the second law of thermodynamics, which appears to govern the case in a metaphorical way.

In Lord Peter Wimsey, the 1975 BBC One production of Dorothy L. Sayers' 1931 novel Five Red Herrings, Thomas is mentioned in a snatch of background dialogue.

He is mentioned in "No Mean City" by A. McArthur and H. Kingsley Long, "Now he insisted on reading extracts from a speech by J. H. Thomas, declaring, moreover, that the railwaymen had never had abler leader" (page 89).

In Ngaio Marsh's Tied Up in Tinsel (1972), a self-made man who clings to his cockney accent says defiantly, "They tell you George V took a shiner to Jimmy Thomas, don't they?