Thomas was training in medicine when the World War II began. He then served in the British Army during the war, in Europe and the Middle East. When he left the army he decided not to return to his medical studies.
He began his film career in 1946 as an editor for Two Cities Films at Denham Studios. His editing work included many films directed by his brother, Ralph. Despite a fairly varied early career, Thomas is now almost exclusively remembered as the director of 30 Carry On films, the innuendo-laden and financially successful British comedy series, produced by Peter Rogers, beginning with Carry On Sergeant in 1958.
Thomas' wife was Barbara; the couple had three daughters. Ralph Thomas's son is Oscar winning film producer Jeremy ThomasCBE. Some of the cast and crew of the Carry On films referred to Thomas as the 'circus ring master' and the 'school headmaster'. He became good friends with many of them over the years.