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.

Mills was promoted to leading roles in A Political Party (1934), a comedy. He was in a series of quota quickies: The River Wolves (1934); Those Were the Days (1934), the first film of Will Hay; The Lash(1934); Blind Justice (1934); Doctor's Orders (1934); and Car of Dreams (1935). He did Jill Darling (1934) on stage and was one of many names in Royal Cavalcade (1935).

Mills had the star role in an A film, Brown on Resolution (1935). It was back to quota quickies for Charing Cross Road (1935) and The First Offence (1936). He had another excellent part in an "A", playing Lord Guildford Dudley in Tudor Rose (1936). He did Aren't Men Beasts? (1936) on stage and worked for Hollywood Director Raoul Walsh in O.H.M.S. (1937).

Mills starred in The Green Cockatoo (1937) directed by william Cameron Menzies. He appeared as Colley in the hugely popular 1939 film version of Goodbye, Mr Chips, opposite Robert Donat.

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.

He was in Men in Shadow (1942) on stage, written by his wife. He achieved acclaim for his performance as an able seaman 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' climb to stardom began when he had the lead role in We Dive at Dawn (1943), a film directed by Asquith about submariners. He was top billed in This Happy Breed (1944), directed by David Lean from a Noël Coward play, and a big hit.

Also popular was Waterloo Road (1945), from Sidney Gilliat, where Mills played a man who goes AWOL to retrieve his wife from draft-dodging Stewart Granger. Mills played a pilot in The Way to the Stars (1945), directed by Asquith from a script by Terence Rattigan, and another big hit in Britain. He did Duet for Two Hands (1945) on stage.

Mills had his greatest success to date in the lead in Great Expectations (1946), directed by David Lean. It was the third biggest hit at the British box office this year and Mills was voted the sixth most popular star.

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.

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. Their combined age was 186 years, a cinema record.

Mills turned Producer with The History of Mr. Polly (1949) from the novel by H. G. Wells. It was directed by Anthony Pelissier and Mills said it was his favorite film. Pelisse also made The Rocking Horse Winner (1949) which Mills produced; he also played a small role. More liked at the box office was a submarine drama, Morning Departure (1950), directed by Baker. By this stage his fee was a reported £20,000 a film.

After Morning Departure Mills took almost two years off. The films he made on his return were not popular: a thriller, Mr. Denning Drives North (1951); The Gentle Gunman (1952), where he and Dirk Bogarde played IRA gunmen for Basil Dearden; The Long Memory (1953), a thriller from Robert Hamer.

Mills' had his first hit in a number of years with Hobson's Choice (1954), directed by Lean. He was in The Colditz Story (1955), a popular war film.

After Escapade (1955), Mills made the popular military comedy The Baby and the Battleship (1956), one of the biggest hits of 1956. Also on that list was another Mills comedy, It's Great to Be Young (1956).

Mills had a key support role as a peasant in War and Peace (1956) and made a cameo in Around the World in 80 Days (1956).

Mills tried some thrillers: Town on Trial (1957) directed by John Guillermin and The Vicious Circle (1957). More liked by the public were war films: Dunkirk (1958), the second most popular film of the year in Britain; Ice Cold in Alex (1958), directed by J. Lee Thompson; and I Was Monty's Double'(1958), directed by Guillermin.

Mills went to Australia to play a cane cutter in the Hollywood financed Summer of the Seventeenth Doll (1959), a disappointing version of the well regarded play.

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

The Rank Organisation insisted Mills play the role of the priest in The Singer Not the Song (1961) opposite Dirk Bogarde, directed by Baker; the film has become regarded as a camp classic. Mills and Baker reteamed on an interracial drama Flame in the Streets (1961) and an Italian-British war film The Valiant (1962).

Mills did a comedy with James Mason, Tiara Tahiti (1962). He had a support role in The Chalk Garden (1964) starring Hayley.

After a cameo on the war film Operation Crossbow (1965), Mills made a third film with his daughter, The Truth About Spring (1965). He had a cameo in King Rat (1965) for Bryan Forbes, who then directed Mills in The Wrong Box (1966). Mills again played Hayley's Father on screen in The Family Way (1966). He then directed her in Sky West and Crooked (1966) from a script written by his wife.

Mills began to drift into character roles, supporting Hugh O'Brian in Africa Texas Style (1967) and Rod Taylor in Chuka (1967). He went to Italy for a giallo, A Black Veil for Lisa (1968) and played william Hamilton in Emma Hamilton (1968).

Mills had a cameo in Oh! What a Lovely War (1969) for Director Richard Attenborough and supported Mark Lester (though was top billed) in Run Wild, Run Free (1969). He went to Australia to star in a convict drama, Adam's Woman (1970).

For his role as the village idiot in Ryan's Daughter (1970) — a complete departure from his usual style – Mills won an Best Supporting Actor Oscar.

He was in Dulcima (1971) then had support roles in Young Winston (1972) for Attenborough, Lady Caroline Lamb (1972), and Oklahoma Crude (1973). On stage he did Veterans at the Royal Court, At the End of the Day (1973), The Good Companions (1974), Great Expectations (1975) and Separate Tables (1977).

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.

He could still get lead roles in films, as shown by The "Human" Factor (1975), Trial by Combat (1976), and The Devil's Advocate (1977). He had good support roles in The Big Sleep (1978) and The Thirty Nine Steps (1978).

On the big screen he was now mainly playing upper crust types as in Zulu Dawn (1979), Gandhi (1982), and Sahara (1983). He did Goodbye Mr Chips on stage (1982) followed by Little Lies (1983).

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).

In the years leading up to his death, he appeared on television only on special occasions, his sight having failed almost completely in 1992. After that, his film roles were brief cameos.

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' 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.

Despite having always previously voted Conservative, Mills publicly supported Tony Blair's Labour Party in the 2001 General Election, later regretting this decision after the invasion of Iraq in 2003.

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 chest infection. His wife died on 1 December 2005. They are buried in Denham Churchyard.