[3][4] He was the younger of two sons of Kenneth Thomas, a barrister who had been a junior at the Nuremberg trials, and his wife, Olga (née Noake).

Thomas made many television appearances, including The Avengers, Coronation Street, Z-Cars, Special Branch, Sutherland's Law, Public Eye, Who Pays the Ferryman?, Bergerac, By the Sword Divided, The Citadel, Knights of God, Boon, London's Burning, Casualty, Taggart, Heartbeat, Sherlock Holmes, How Green Was My Valley, Torchwood, Children of the Stones, Star Maidens, and Midsomer Murders.

His appearances include RSC productions of Twelfth Night, Othello and Anna Christie; English Shakespeare Company productions of Henry IV, Part 1 and Part 2 and Henry V; and King Lear, Educating Rita, Cat on a Hot Tin Roof, The Crucible, Equus and Déjà Vu.

From 1999 to 2005, he took part in 12 out of 14 CD episodes of MJTV's original audio comedy sci-fi drama series Soldiers of Love, in which he played camp Welsh TV host Hywel Hammond and villain of the piece Aaron Arkenstein.

In 2013 he appeared as Brother Cadfael in Middle Ground Theatre Company's adaption of The Virgin in the Ice by Ellis Peters.

[8] His Blake's 7 co-star Stephen Greif paid tribute, saying he was "very sad to hear of the death of my good friend Gareth Thomas.