Mills took an early interest in acting, making his professional début at the London Hippodrome in The Five O’Clock Girl in 1929. He followed this with a cabaret act.

On his return Mills starred in The 1931 Revue, Coward’s Cavalcade (1931) and the Noël Coward revue Words and Music (1932).

His first wife was the actress Aileen Raymond who died only five days after he did. They were married in 1932 and divorced in 1941. Raymond later became the mother of actor Ian Ogilvy.

In September 1939, at the start of the Second World War, Mills enlisted in the British Army, joining the Royal Engineers. He was later commissioned as a Second Lieutenant, but in 1942 he received a medical discharge because of a stomach ulcer.

At the Old Vic he was in A Midsummer Night’s Dream (1939), She Stoops to Conquer (1939) and Of Mice and Men (1939–40). He joined the army in 1939 but occasionally made films on leave. He went back to movies with Old Bill and Son (1940) and made Cottage to Let (1941), a war film for Anthony Asquith. Mills went back to supporting Will Hay in The Black Sheep of Whitehall (1942) and he was one of many names in the war film, The Big Blockade (1942).

His second wife was the dramatist Mary Hayley Bell. Their marriage, on 16 January 1941, lasted for 64 years, until his death in 2005. They were married in a rushed civil ceremony, because of the war; it was not until 60 years later that they were married in a church. They lived in The Wick, London, for many years. They sold the house to musician Ronnie Wood in 1971 and moved to Hills House, Denham, south Buckinghamshire.

He was in Men in Shadow (1942) on stage, written by his wife. He achieved acclaim for his performance as an able seaman in Noël Coward’s In Which We Serve (1942), a huge hit. Mills had another good support role in The Young Mr. Pitt (1942) playing William Wilberforce opposite Robert Donat. He was invalided out of the army in 1942.

Mills and Bell had two daughters, Juliet, star of television’s Nanny and the Professor and Hayley, a Disney child star who appeared in Pollyanna, The Parent Trap and Whistle Down the Wind, and one son, Jonathan Mills, a screenwriter. In 1947, Mills appeared with his daughters in the film So Well Remembered. The three also appeared together decades later, on an episode of ABC’s The Love Boat. Mills’s grandson by Hayley, Crispian Mills, is a musician, best known for his work with the raga rock group Kula Shaker.

In the 1959 crime drama Tiger Bay, directed by Thompson, Mills played a police detective investigating a murder that a young girl has witnessed. His daughter Hayley was cast, and earned excellent reviews.

He was the subject of This Is Your Life on two occasions, firstly in 1960 when he was surprised by Eamonn Andrews outside Pinewood Studios, and again in 1983 when Eamonn surprised him on the stage of London’s Wyndham’s Theatre at the curtain call of the play Little Lies.

Mills was appointed a Commander of the Order of the British Empire (CBE) in 1960. In 1976 he was knighted by the Queen.

Also on the small screen, in 1974 he starred as Captain Tommy “The Elephant” Devon in the six-part television drama series The Zoo Gang, about a group of former underground freedom fighters from World War II, with Brian Keith, Lilli Palmer and Barry Morse.

His most famous television role was probably as the title character in Quatermass for ITV in 1979. He followed this with a sitcom in Young at Heart (1980–82).

In 1986 he did The Petition at the National and the following year did Pygmalion on Broadway. He provided a voice for When the Wind Blows (1986) and supported Madonna in Who’s That Girl (1987). His best roles were on TV in Harnessing Peacocks (1993) and Martin Chuzzlewit (1994). Mills also starred as Gus: The Theatre Cat in the filmed version of the musical Cats in 1998.

In 1999, at 91 years of age, Mills became the oldest joining member of the entertainment charitable fraternity, the Grand Order of Water Rats.

In 2000, Mills released his extensive home cine-film footage in a documentary film entitled Sir John Mills’s Moving Memories, with interviews with Mills, his children Hayley, Juliet and Jonathan and Richard Attenborough. The film was produced and written by Jonathan Mills, directed and edited by Marcus Dillistone, and features behind the scenes footage and stories from films such as Ice Cold in Alex and Dunkirk. In addition the film also includes home footage of many of Mills’s friends and fellow cast members including Laurence Olivier, Harry Andrews, Walt Disney, David Niven, Dirk Bogarde, Rex Harrison and Tyrone Power. Mills’s last cinema appearance was playing a tramp in Lights 2 (directed by Marcus Dillistone); the cinematographer was Jack Cardiff. They had last worked together on Scott of the Antarctic in 1948.

Despite having always previously voted Conservative, Mills publicly supported Tony Blair’s Labour Party in the 2001 General Election.

In 2002, he received a Fellowship of the British Academy of Film and Television Arts (BAFTA), their highest award, and was named a Disney Legend by the Walt Disney Company.

He died aged 97 on 23 April 2005 in Denham, Buckinghamshire, following a stroke. His wife died on 1 December 2005. They are buried in St Mary the Virgin Churchyard, Denham.