Born in Hull, East Riding of Yorkshire, England, Thomas was educated in Bristol and London, and was training in medicine when World War II began.
[2] He served four years in the British Army during the war, and upon his return to civilian life thought it too late to continue his medical studies.
[2] He began his movie career at Denham Studios, eventually becoming an assistant film editor beginning with Laurence Olivier's Hamlet (1948).
His editing work included many films directed by his older brother, Ralph Thomas.
Other works as a director include the comedy Please Turn Over (1959) and a drama of post-war Austria, The Second Victory (1986).