From 1961 to 1966, he starred in the sitcom Marriage Lines with Richard Briers and Prunella Scales. De Souza had roles in the Hammer films The Phantom of the Opera and The Kiss of the Vampire (both 1962). In the same year he appeared in “Six Hands Across the Table”, an episode of British television series The Avengers.

De Souza appeared as the lead in the Doctor Who story Mission to the Unknown (1965) – the only story ever broadcast in the series not to feature the Doctor in any capacity. In 1977, he played Sheik Hosein in the James Bond film The Spy Who Loved Me. He was solicitor Bonny Bernard in the first series of Rumpole of the Bailey (1978). In the same year he appeared in “Hearts and Minds”, the last episode of The Sweeney to be filmed, which also featured the popular comedians Morecambe and Wise. In 1982, he appeared in the final Sapphire & Steel adventure as “The Man”. He also appeared in the television version of After Henry (1989–90), Farrington and, earlier, took the part of Soveral (the Portuguese Ambassador to Britain) in Edward the Seventh (1975). He had a cameo appearance in the 2007 fantasy film The Golden Compass as the Second High Councillor. In succession to the originator of the role, Valentine Dyall, he was The Man in Black on BBC Radio 4 between 1988 and 1992. De Souza played the role of Afonso in One Foot in the Grave One Foot in the Algarve (1993).

He joined the British soap opera Coronation Street as Colin Grimshaw, where he made his first appearance on 12 December 2008. His character died in May 2009.