Donald originally intended to be a teacher, but seeing Sir Cedric Hardwicke and Dame Edith Evans in The Late Christopher Bean made him decide to be an actor.

He appeared in Twelfth Night with Michael Redgrave and understudied John Gielgud in King Lear.

He joined the RASC before being assigned to British Army Intelligence where he typed up decoded enemy messages.

On stage he was in The Eagle with Two Heads (1947) and You Never Can Tell (1948) In films, MGM loaned him to Gainsborough Studios for Broken Journey (1948).

Donald had the lead in a comedy Brandy for the Parson (1952) and supported Trevor Howard and Richard Attenborough in Gift Horse (1952).

It was Donald's voice that read aloud the famous letters from the artist, played by Kirk Douglas, to his brother, which formed the narrative backbone of the film.

He portrayed Major Clipton, the doctor who expresses grave doubts about the sanity of Colonel Nicholson's (Alec Guinness) efforts to build the bridge in order to show up his Japanese captors, in the war film The Bridge on the River Kwai (1957).

Donald was in much demand to play supporting roles in action and prisoner-of-war films: The Vikings (1958); Third Man on the Mountain (1959); Group Captain Ramsey, the Senior British Officer in The Great Escape (1963); King Rat (1965), a doctor in a POW camp; and Cast a Giant Shadow (1966).

Later film roles included Hannibal Brooks (1969), The Royal Hunt of the Sun (1969), David Copperfield (1969), Conduct Unbecoming (1975) and The Big Sleep (1978).