Howard was born in Cliftonville, Kent, England, the son of Mabel Grey (Wallace) and Arthur John Howard. His Father was an insurance underwriter for Lloyds of London and he spent the first eight years of his life travelling around the world. He was educated at Clifton College (to which he left in his will a substantial legacy for a drama scholarship) and at the Royal Academy of Dramatic Art (RADA). In 1933, at the end of his first year, he was chosen as best actor in his class for his performance as Benedict in a school production of Much Ado About Nothing. While Howard was still studying, he made his professional debut at the Gate Theatre in Revolt in a Reformatory (1934).

After a theatrical role in The Recruiting Officer (1943), Howard began working in films with an uncredited part The Way Ahead (1944), directed by Carol Reed. He was in a big stage hit, A Soldier for Christmas (1944) and a production of Eugene O'Neill's Anna Christie (1944). Howard received his first credit for The Way to the Stars (1945), playing a pilot.

Howard's performance in The Way Ahead came to the attention of David Lean, who was looking for someone to play the role of Alec in Brief Encounter (1945). Lean recommended him to Noël Coward, who agreed with the suggestion, and the success of the film launched Howard's film career.

He followed it with I See a Dark Stranger (1946) with Deborah Kerr, and Green for Danger (1947), starring Alastair Sim. Both films were successful as was They Made Me a Fugitive (1947). That year British exhibitors voted Howard the 10th most popular British star at the box office. So Well Remembered (1948) was made with American talent and money and was a hit in Britain but lost money overall.

Howard was reunited with Lean for The Passionate Friends (1949), but the film was not a success. However The Third Man (1949), which Howard starred in alongside Orson Welles and Joseph Cotten for Carol Reed from a story by Graham Greene, was a huge international success, and became the film of which Howard was most proud. During filming in Vienna, Howard was keen to get to his favourite bar for a drink as soon as filming had finished for the evening. On one occasion, Howard was in too much of a hurry to change out of his uniform as a British Army major. After a few drinks, he got into an argument and attracted the attention of a real major, who ordered the Military Police to arrest Howard as an impostor. Howard was forced to apologise and was summoned to appear before the British commanding general, Sir Alexander Galloway.

Howard was the lead in Golden Salamander (1950) and played Peter Churchill in Odette (1950) with Anna Neagle, a big hit in Britain. It was directed by Herbert Wilcox who put Howard under contract. He loaned Howard to Betty Box and Ralph Thomas to make The Clouded Yellow (1950), a popular thriller with Jean Simmons. These films helped Howard be voted the 2nd biggest British star at the box office in 1951 and the 5th biggest (and eleventh bigger over-all) in 1951.

Howard was reunited with Carol Reed for Outcast of the Islands (1952) and he made a war film, Gift Horse (1952). That year he made his final appearance in Britain's ten most popular actors, coming in at number nine. He was in another adaptation of a Graham Greene story, The Heart of the Matter (1953). Greene also wrote and produced Howard's next film, the British-Italian The Stranger's Hand (1954). Howard was in a French movie, The Lovers of Lisbon (1955), then supported Jose Ferrer in a war film from Warwick Pictures, The Cockleshell Heroes (1955), which was popular in Britain.

Howard's first Hollywood film was Run for the Sun (1956), where he played a villain to Richard Widmark's hero. He made a cameo in Around the World in 80 Days (1956) and again played a villain to an American star, Victor Mature, in Warwick's Interpol (1957).

Howard starred in Manuela (1957) then supported william Holden in Carol Reed's The Key (1958), for which he received the Best Actor award from the British Academy of Film and Television Arts. When william Holden dropped out of the lead of The Roots of Heaven (1958), Howard stepped in - the star part in a Hollywood film (although top billing went to Errol Flynn).

Howard recorded two Shakespeare performances, the first, recorded in the 1960s, was as Petruchio opposite Margaret Leighton's Kate in Caedmon Records' complete recording of The Taming of the Shrew; the second was in the title role of King Lear for the BBC World Service in 1986.

Howard was reunited with Holden for The Lion (1962). He was Captain Bligh to Marlon Brando's Fletcher Christian in Mutiny on the Bounty (1962). He was in a production of Hadda Gabbler (1962) US TV and played the Victorian prime minister in The Invincible Mr Disraeli (1963), an episode of the Hallmark Hall of Fame for which he won an Emmy award for his role then supported Robert Mitchum in Man in the Middle (1964) and Cary Grant in Father Goose (1964). After a cameo in Operation Crossbow (1965), Howard supported Frank Sinatra in Von Ryan's Express (1965), Brando and Yul Brynner in Morituri (1965), and Rod Taylor in The Liquidator (1965). After a cameo in The Poppy Is Also a Flower (1966) he made two with Brynner, Triple Cross (1966) and The Long Duel (1967).

Howard had a change of pace supporting Hayley Mills in Pretty Polly (1968). He went back to military roles: The Charge of the Light Brigade (1968), as Lord Cardigan, and Battle of Britain (1969). He had support parts in Lola (1969) and Ryan's Daughter (1970), the latter for David Lean.

He made a Swedish film The Night Visitor (1971) then settled into a career as a character actor: To Catch a Spy (1971), supporting Kirk Douglas; Mary, Queen of Scots (1971), as Sir william Cecil; Kidnapped (1971); Pope Joan (1972); Ludwig (1972); The Offence (1972), with Sean Connery; A Doll's House (1973), for Joseph Losey; Who? (1974), supporting Elliott Gould; and Catholics (1974) for British TV.

He was in some horror films - Craze (1974), Persecution (1974) - and the more prestigious 11 Harrowhouse (1974). In The Count of Monte Cristo (1975) he mentored Richard Chamberlain. He was military men in Hennessy (1975) and Conduct Unbecoming (1975). Around this time he complained that he had to work so hard because of the high rate of tax in Britain.

Howard could be found in Albino (1976), shot in Rhodesia; The Bawdy Adventures of Tom Jones (1976); Aces High (1976); Eliza Fraser (1976), shot in Australia; The Last Remake of Beau Geste (1977); and Stevie (1978). He was one of many names in Superman (1978), Hurricane (1979), Meteor (1979) and The Sea Wolves (1980). He appeared in a TV series Shillingbury Tales (1980-81) and had a rare lead in Sir Henry at Rawlinson End (1980).

Howard was also top billed in Windwalker (1981). He and Celia Johnson from Brief Encounter were reunited in Staying On (1980) for British TV.

Howard managed to appear in some prestigious movies towards the end of his career: The Deadly Game (1982), The Missionary (1982), Gandhi (1982), George Washington (1984), Shaka Zulu (1986), Dust (1985) and Peter the Great (1986).

He died on 7 January 1988 from hepatic failure and cirrhosis of the liver in Arkley, Barnet, aged 74, and was survived by his widow Helen Cherry.

Although stories of his courageous wartime Service in the British Army's Royal Corps of Signals earned him much respect among fellow actors and fans alike, files held in the Public Record Office reveal that he had actually been discharged from the British Army in 1943 for mental instability and having a "psychopathic personality". The story, which surfaced in Terence Pettigrew's biography of the actor, published by Peter Owen in 2001, was initially denied by Howard's widow, Actress Helen Cherry. Later, confronted with official records, she told The Daily Telegraph (24 June 2001) that his mother had claimed he was a holder of the Military Cross. She added that Howard had an honourable military record and "had nothing to be ashamed of".

A British government document leaked to the Sunday Times in 2003 shows that Howard was among almost 300 individuals to decline official honours. He declined a CBE in 1982.