Born as Albert Van Ecke in Brooklyn, New York, he adopted his mother's maiden name of Dekker as his stage name. Dekker attended Bowdoin College and made his professional acting debut with a Cincinnati stock company in 1927. Within a few months, Dekker was featured in the Broadway production of Eugene O'Neill's play Marco Millions.
On April 4, 1929, Dekker married actress Esther Guernini. The couple had two sons and a daughter before divorcing.
Dekker as Dr. Alexander Thorkel in the 1940 film Dr. CyclopsAfter a decade of theatrical appearances, Dekker transferred to Hollywood in 1937, and made his first film, 1937's The Great Garrick. He spent most of the rest of his acting career in the cinema, but also returned to the stage from time to time.
He replaced Lee J. Cobb as Willy Loman in the original production of Arthur Miller's Death of a Salesman, and during a five-year stint back on Broadway in the early 1960s, he played the Duke of Norfolk in Robert Bolt's A Man for All Seasons.
Dekker appeared in some seventy films from the 1930s to 1960s, but his four most famous screen roles were as a mad scientist in the 1940 horror film Dr. Cyclops, as a vicious hitman in the The Killers, as a dangerous dealer in atomic fuel in the 1955 film noir Kiss Me Deadly, and as an unscrupulous railroad detective in Sam Peckinpah's western The Wild Bunch. He was rarely cast in romantic roles, but in the film Seven Sinners, featuring a romance between Marlene Dietrich and John Wayne, Dietrich sails off with Dekker's character at the end of the film. Dekker's role as Pat Harrigan in The Wild Bunch would be his last screen appearance.
On May 5, 1968, Dekker was found dead in his Hollywood home by his fiancée Geraldine Saunders after failing to answer numerous phone calls for two days. He was found naked, kneeling in his bathtub with a noose wrapped around his neck that was looped around the shower's curtain rod. He was also handcuffed, blindfolded, and had sexually explicit words scrawled on his body in red lipstick. There were no signs of forced entry, but money and camera equipment were missing from Dekker's home. He was 62 years old.
He was interred at Garden State Crematory in North Bergen, New Jersey.
Dekker's off-screen preoccupation with politics led to his winning a seat in the California State Assembly for the 57th Assembly District in 1944. Dekker served as a Democratic member for the Assembly until 1946.
During the McCarthy era he was an outspoken critic of U.S. Senator Joseph McCarthy's tactics; to avoid being blacklisted he spent most of the blacklist working on Broadway rather than Hollywood.