Zucco was born in Manchester, Lancashire, on 11 January 1886. His mother, Marian (née Rintoul), and ran a dressmaking business; it is claimed she was a former lady-in-waiting to Queen Victoria - but this is untrue as the honour was only accessible to titled ladies of high rank (duchesses, marchionesses, countesses, viscountesses, and baronesses). His father, George De Sylla Zucco, was a Greek merchant.
Zucco debuted on the Canadian stage in 1908. He and his wife Frances toured the American vaudeville circuit during the 1910s, their satirical sketch about suffragettes earning them renown.
He returned to the UK and served as a lieutenant in the British Army's West Yorkshire Regiment during the First World War. He became a leading stage actor of the 1920s, and made his film debut as Eugène Godefroy Cavaignac in The Dreyfus Case (1931), a British film dramatising the Dreyfus Affair.
He and his wife had a daughter, Frances, and a grandson, George Zucco (né Canto).
Zucco returned to the United States in 1935 to play Benjamin Disraeli alongside Helen Hayes in Victoria Regina, and appeared with Gary Cooper and George Raft in Souls at Sea (1937). He played Professor Moriarty in The Adventures of Sherlock Holmes (1939), opposite Basil Rathbone as Sherlock Holmes and Nigel Bruce as Dr. Watson. Zucco earned a reputation as a bespectacled, nefarious character in films such as After the Thin Man, Fast Company, Arrest Bulldog Drummond, Charlie Chan in Honolulu, The Cat and the Canary, and My Favorite Blonde. During the 1940s, he took every role he was offered, landing himself in B-films and Universal horror films, including The Mummy's Hand (1940), The Mummy's Tomb (1942), The Mad Monster (1942), The Mad Ghoul (1943), Dead Men Walk (1943), The Mummy's Ghost (1944), House of Frankenstein (1944), and Tarzan and the Mermaids (1948). He was reunited with Basil Rathbone in another Sherlock Holmes adventure, Sherlock Holmes in Washington, this time playing not Moriarty, but a Nazi spy.
He retired due to illness, after playing a bit part in David and Bathsheba (1951). Zucco was to have played in The Desert Fox: The Story of Rommel, but his health issues resulted in his being replaced by Cedric Hardwicke.
Zucco suffered from dementia throughout the fifties and died on 27 May 1960 from pneumonia in an assisted-living facility at the age of 74.
His daughter, Frances (1931-1962), died of throat cancer at age 30, and his widow died from natural causes in 1999 (at age 99).
Kenneth Anger, in his 1988 book Hollywood Babylon II, claimed that Zucco had died in a madhouse, convinced that he was being haunted by H.P. Lovecraft's creation Cthulhu, and that Zucco's wife and daughter had committed suicide in response to the loss. Zucco's widow was still alive and well when Anger's book was published, and she later gave an interview to Filmfax magazine about Anger's erroneous claims.