Guinness was born in Paddington, London as Alec Guinness de Cuffe. Under the column for name (where the first names only are usually stated) his birth certificate says 'Alec Guinness'. There is nothing written in the column for name and surname of father. In the column for mother's name is written 'Agnes de Cuffe'. On this basis it has been frequently speculated that the actor's father was a member of the Irish Guinness family.

However, his benefactor was a Scottish banker named Andrew Geddes, and the similarity of his name to the name written on the actor's birth certificate ('Alec Guinness') may be a subtle reference to the identity of the actor's father.

From 1875, English law required both the presence and consent of the father when the birth of an illegitimate child was registered in order for his name to be put on the certificate. His mother's maiden name was Agnes Cuff (born 8 December 1890), daughter of Edward Cuff and wife Mary Ann Cuff Benfield.

She would later marry a shell shocked veteran of the Anglo-Irish War who, according to Guinness, hallucinated that his own closets were filled with Sinn Féin gunmen waiting to kill him.

The man who believed he was Alec Guinness' biological father, Andrew Geddes, paid for the actor's private school education, but the two never met and the identity of his father continues to be debated.

Guinness first worked writing copy for advertising before making his debut at the Albery Theatre in 1936 at the age of 22, playing the role of Osric in John Gielgud's wildly successful production of Hamlet.

During this time he worked with many actors and actresses who would become his friends and frequent co-stars in the future, including John Gielgud, Peggy Ashcroft, Anthony Quayle, and Jack Hawkins. An early influence from afar was Stan Laurel, whom Guinness admired.

Guinness continued playing Shakespearean roles throughout his career. In 1937 he played the role of Aumerle in Richard II and Lorenzo in The Merchant of Venice under the direction of John Gielgud. He starred in a 1938 production of Hamlet which won him acclaim on both sides of the Atlantic.

He also appeared as Romeo in a production of Romeo and Juliet (1939), Andrew Aguecheek in Twelfth Night and as Exeter in Henry V in 1937, both opposite Laurence Olivier, and Ferdinand in The Tempest, opposite Gielgud as Prospero.

In 1939, he adapted Charles Dickens' novel Great Expectations for the stage, playing the part of Herbert Pocket. The play was a success. One of its viewers was a young British film editor named David Lean, who had Guinness reprise his role in the former's 1946 film adaptation of the play.

Guinness served in the Royal Navy Volunteer Reserve in World War II, serving first as a seaman in 1941 and being commissioned the following year. He commanded a landing craft taking part in the invasion of Sicily and Elba and later ferried supplies to the Yugoslav partisans.

During the war, he appeared in Terence Rattigan's West End Play for Bomber Command, Flare Path. He returned to the Old Vic in 1946 and stayed until 1948, playing Abel Drugger in Ben Jonson's The Alchemist, the Fool in King Lear opposite Laurence Olivier in the title role, DeGuiche in Cyrano de Bergerac opposite Ralph Richardson in the title role, and finally starring in an Old Vic production himself as Shakespeare's Richard II.

After leaving the Old Vic, he had a success as the Uninvited Guest in the Broadway production of T. S. Eliot's The Cocktail Party (1950, revived at the Edinburgh Festival in 1968), but his second attempt at the title role of Hamlet, this time under his own direction at the New Theatre (1951), proved a major theatrical disaster.

He was initially mainly associated with the Ealing comedies, and particularly for playing eight different characters in Kind Hearts and Coronets. Other films from this period included The Lavender Hill Mob, The Ladykillers, and The Man in the White Suit. In 1952, director Ronald Neame cast Guinness in his first romantic lead role, opposite Petula Clark in The Card.

Invited by his friend Tyrone Guthrie to join in the premier season of the Stratford Festival of Canada, Guinness lived for a brief time in Stratford, Ontario. On July 13, 1953, Guinness spoke the first lines of the first play produced by the festival (Shakespeare's Richard III): "Now is the winter of our discontent/Made glorious summer by this son of York."

Guinness won particular acclaim for his work with director David Lean. After appearing in Lean's Great Expectations and Oliver Twist, he was given a starring role opposite William Holden in Bridge on the River Kwai. For his performance as Colonel Nicholson, the unyielding British POW leader, Guinness won an Academy Award for Best Actor.

Despite a difficult and often hostile relationship, Lean, referring to Guinness as "my good luck charm", continued to cast Guinness in character roles in his later films: Arab leader Prince Feisal in Lawrence of Arabia; the title character's half-brother, Bolshevik leader Yevgraf, in Doctor Zhivago; and Indian mystic Godbole in A Passage to India. He was also offered a role in Lean's adaptation of Ryan's Daughter (1970), but declined.

Other famous roles of this time period included The Swan (1956) with Grace Kelly in her last film role, The Horse's Mouth (1958) in which Guinness played the part of drunken painter Gulley Jimson as well as contributing the screenplay, for which he was nominated for an Academy Award for Best Writing, Screenplay Based on Material from Another Medium, Tunes of Glory (1960), Damn the Defiant! (1962), The Fall of the Roman Empire (1964), The Quiller Memorandum (1966), Scrooge (1970), and the title role in Hitler: The Last Ten Days (1973) (which he considered his best film performance).

Guinness turned down roles in many well-received films - most notably The Spy Who Came in From the Cold - for ones that paid him better, although he won a Tony Award for his Broadway triumph as poet Dylan Thomas in Dylan. He followed this success up by playing the title role in Macbeth opposite Simone Signoret at the Royal Court Theatre in 1966, one of the most conspicuous failures of his career.

From the 1970s, Guinness made regular television appearances, including the part of George Smiley in the serializations of two novels by John le Carré: Tinker, Tailor, Soldier, Spy and Smiley's People. Le Carré was so impressed by Guinness's performance as Smiley that he based his characterization of Smiley in subsequent novels on Guinness. One of his last appearances was in the acclaimed BBC drama Eskimo Day.

Guinness received his fifth Oscar nomination for his performance in Charles Dickens' Little Dorrit in 1989. He received an honorary Oscar in 1980 "for advancing the art of screen acting through a host of memorable and distinguished performances."