Portman joined the company as a 'passenger' and appeared in their production of Richard II at the Victoria Hall, Sunderland which led to Courtneidge giving him a contract.

Portman made his West End debut at the Savoy Theatre in London, in September 1924, as Antipholous of Syracuse in The Comedy of Errors.

He also made Hyde Park Corner with Gordon Harker and directed by Sinclair Hill; Old Roses and Abdul the Damned.

In the semi-autobiographical play Dinner with Ribbentrop by screenwriter Norman Hudis, a former personal assistant to Portman, Hudis relates a claim made often by Portman that in 1937, before the start of the Second World War, he had had dinner in London with Joachim von Ribbentrop (then the German Ambassador to Britain).

[4][5] In 1941 he had his first important film role playing Lieutenant Hirth, a Nazi on the run, in Powell and Pressburger's 49th Parallel, which was a big hit in the US and Britain.

[6] Portman was in Powell and Pressburger's follow up, One of Our Aircraft Is Missing (1942), which reworked the story of The 49th Parallel to be about Allied pilots in occupied Holland.

He made two films for the new producing team of Maxwell Setton and Aubrey Baring, The Spider and the Fly (1949) and Cairo Road (1950).

Portman was one of many names in The Magic Box (1951) and then made an Ealing comedy, His Excellency (1952), playing a trade unionist who becomes Governor of a British colony.

For Baring and Setton, he made South of Algiers (1953) then had a big hit on stage in Terence Rattigan's Separate Tables and on film in The Colditz Story (1955).

In contrast, Flowering Cherry by Robert Bolt, with Portman in the title role, only lasted five performances on Broadway.

Later film roles included in The Naked Edge (1961), Freud: The Secret Passion (1962), West 11 (1963), The Man Who Finally Died (1963), The Bedford Incident (1965), and The Spy with a Cold Nose (1966).