Mason won his first professional role in The Rascal and made his debut in London's West End theater world in 1933 with Gallows Glorious. A year after he joined London's Old Vic theater, he made his screen debut in Late Extra in 1935.

He became a regular British screen presence in late '30s "quota quickies," including The High Command (1937). The actor made a career and personal breakthrough, however, with I Met a Murderer (1939). Along with co-writing, co-producing, and starring in the film, he also wound up marrying his leading lady, Pamela Kellino, in 1940. In 1942 he starred in the War movie, Secret Mission.

Mason, who was just under six feet tall, became Britain's biggest screen star shortly after with his performance as the sadistic title character in the Gainsborough Studios melodrama The Man in Grey (1943). He cemented his fame as the cruel romantic leads women loved in the critically weak, but highly popular, Gainsborough costume dramas Fanny by Gaslight (1944) and The Wicked Lady (1945), finally achieving international stardom for his charismatic performance as Ann Todd's cane-wielding mentor in the well-received The Seventh Veil (1946).

Rather than immediately going to Hollywood, however, Mason remained in England. Revealing that he could be more than just brutal leading men in weepy potboilers, he added an artistic as well as popular triumph to his credits with Carol Reed's Odd Man Out (1947). Starring Mason as a doomed IRA leader hunted by the police, Odd Man Out garnered international raves, and he often cited it as his favorite among his many films.

After co-starring in the British drama The Upturned Glass (1947), the Masons and their 12 cats finally headed to Hollywood (via a stint on Broadway in Bathsheba) in 1947. Spurning a long-term studio contract, Mason became one of Hollywood's busiest free agents. Anxious not to be typecast, he bucked his image as the irresistible sadist by playing trapped wife Barbara Bel Geddes' kind boss in Max Ophüls' Caught and appearing as Gustave Flaubert in Vincente Minnelli's version of Madame Bovary (both 1949). Mason returned to roguish form (albeit tempered by sympathy) with his second Ophüls film, The Reckless Moment. As smooth Irish hood Donnelly, Mason moved from venal blackmailer bedeviling Joan Bennett's anguished mother to her compassionate ally, adding emotional depth to the film's noir atmosphere.

Mason's American career was firmly established by his late-'40s successes, and his elegant range helped him remain a Hollywood fixture throughout the '50s. Along with two superb turns as wily, disillusioned German Field Marshal Rommel in The Desert Fox (1951) and The Desert Rats (1953), Mason also engaged in a glorious Technicolor romance with Ava Gardner in Pandora and the Flying Dutchman (1951) and played the villain in the swashbuckler The Prisoner of Zenda (1952). Calling on his suave intelligence, Mason starred as cool butler-turned-spy Cicero in what he considered his best Hollywood film, Joseph L. Mankiewicz's smart espionage thriller 5 Fingers (1952). The actor quickly reunited with Mankiewicz to play the treasonous Brutus in the director's excellent Shakespeare-adaptation Julius Caesar in 1953.

Taking a brief break from Hollywood, Mason returned to Europe to write and produce the British drama The Lady Possessed (1952), co-starring his wife, and star as a Harry Lime-esque black marketer in Carol Reed's The Man Between (1953). Mason stepped behind the camera as director for the first and only time with the subsequent short film The Child (1954), featuring his wife and daughter Portland Mason.

Returning to Hollywood acting, Mason garnered numerous accolades for George Cukor's lavish 1954 remake of A Star Is Born. Though the drama of his co-star Judy Garland's "comeback" and the studio's decision to re-cut the film after its debut threatened to overshadow its content, Mason's sublimely controlled fury and anguish as doomed falling star Norman Maine still brought him high praise and earned him his only Best Actor Academy Award nomination. Whether because he never particularly liked the film or because he wasn't a great fan of the Hollywood system, Mason dismissed the Oscar hoopla, noting:

"They don't mean anything unless you win one; then your salary goes up."

Still, 1954 proved to be a banner year for the actor, as his artistic triumph in A Star Is Born was accompanied by the popular screen version of Jules Verne's 20,000 Leagues Under the Sea (1954), featuring Mason as megalomaniac submarine skipper Captain Nemo. Bolstered by these successes, he used his clout to produce and star in Nicholas Ray's tough, groundbreaking family drama Bigger Than Life (1956). Featuring Mason as a mild-mannered father who becomes disastrously hooked on cortisone, Bigger Than Life was one of the first Hollywood movies to examine prescription drug abuse; its bold subject matter, however, was box-office poison.

Soured on producing, Mason focused solely on acting for the latter half of the decade, playing such roles as a plantation owner in Island in the Sun (1957), a psychopath's unwilling accomplice in Cry Terror! (1958), an adventurer in Journey to the Center of the Earth (1959), and, most notably, Cary Grant's velvety nemesis Van Dam in Alfred Hitchcock's masterpiece North by Northwest (1959).

Edging away from Hollywood, Mason took a supporting role in the British drama The Trials of Oscar Wilde in 1960. Having retained his British citizenship during his years in America, he left Hollywood permanently two years later, relocating to Switzerland with his family. After the move, Mason took on the challenge of playing agonized pedophile Humbert Humbert in Stanley Kubrick's 1962 adaptation of Vladimir Nabokov's novel Lolita. Whether duping clueless mother Shelley Winters into marriage, lusting after her teenage daughter Sue Lyon, or helplessly pursuing rival pervert Peter Sellers, Mason's Humbert was as much broken victim as scheming predator, injecting uneasy emotion into the difficult role.

Following an acrimonious divorce from Pamela and an expensive settlement in 1964, Mason started working non-stop, segueing into mostly supporting roles in British, American, and European productions. Despite appearing in such dubious fare as Genghis Khan (1965) and The Yin and Yang of Dr. Go (1971), Mason continued to resist typecasting with his strong turn as a lecherous friend in The Pumpkin Eater (1964), and distinguished himself in such films as Anthony Mann's sword-and-sandal epic The Fall of the Roman Empire (1964) and the adaptation of Lord Jim in 1965.

Showing his facility with lighter films, Mason earned a Best Supporting Actor Oscar nomination for his performance as ugly duckling Lynn Redgrave's older sugar daddy in the romantic comedy Georgy Girl (1966). Beginning a collaboration that would last until the end of his career, Mason followed that film with his first for director Sidney Lumet, playing a George Smiley-esque British spy in the exemplary John Le Carré adaptation The Deadly Affair (1967). Making the most of the actor's abilities, Lumet subsequently cast him as a 19th century Russian in his screen version of Chekhov's The Sea Gull in 1968, and called upon Mason when he needed a Catholic schoolteacher for his 1972 adaptation of Child's Play.

Amid all this work, Mason met his second wife Clarissa Kaye on the set of Michael Powell's Australian romp Age of Consent (1969) and married her in 1971. With Kaye putting Mason ahead of her career, the actor maintained his prolific pace, starring in the skillful murder mystery The Last of Sheila (1973), playing Magwitch in a TV version of Great Expectations in 1974, appearing as an estate patriarch in the humid potboiler Mandingo (1975), a Cuban minister in the pre-Holocaust drama Voyage of the Damned (1976), and a weathered German colonel in Sam Peckinpah's only war film, Cross of Iron (1976).

Mason's inimitable air of gravitas suited the role of Joseph of Arimathea in the made-for-TV film Jesus of Nazareth (1977), and enhanced the humor of his appearance as the God-like Mr. Jordan in Warren Beatty's highly popular romantic fantasy Heaven Can Wait (1978).