Dekker was born in Brooklyn, New York, the only child of Thomas and Grace Ecke Van Dekker. He attended Richmond Hill High School, where he appeared in stage productions. He then attended Bowdoin College, where he majored in pre-med with plans to become a Doctor. On the advice of a friend, he decided to pursue acting as a career instead. He 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.
Dekker appeared in some seventy films from the 1930s to the 1960s, but his four most famous screen roles were as a mad scientist in the 1940 horror film Dr. Cyclops, as a Criminal mastermind in 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. In 1959 he played a convincing Texas Ranger Captain Rucker in The Wonderful Country. 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.
After 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.
Dekker's off-screen interest in 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.
In April 1957, Dekker's 16-year-old son, John, shot himself with a .22 rifle at the family's home in Hastings-on-Hudson, New York. He had reportedly been working on a silencer for the rifle for a year. His death was ruled accidental.
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.
On May 5, 1968, Dekker was found dead in his Hollywood home by his fiancée, fashion model and Future Love Boat creator Jeraldine Saunders. He was naked, kneeling in the bathtub, with a noose tightly wrapped around his neck and looped around the shower curtain rod. He was blindfolded, his wrists were handcuffed, there was a ball gag in his mouth, and two hypodermic needles were inserted in one arm. His body was covered in explicit words and drawings in red lipstick.
