Keel was born Harold Clifford Leek in 1919 to a poor family in Illinois; his first job in Los Angeles was as a singing busboy. He also worked as a manufacturing representative for Douglas Aircraft before he began to study singing in earnest at age 20. In addition to his several starring roles on the London stage, including the first West End production of Oklahoma! (in which he was billed as Harold Keel), he was a replacement performer in the original Broadway productions of Oklahoma!, Carousel, and No Strings, and he played Billy Bigelow in the 1957 City Center revival of Carousel. He created leading roles in the short-lived Broadway musicals Saratoga and Ambassador, and he later toured in shows such Show Boat, South Pacific, Man of La Mancha, I Do! I Do!, and On a Clear Day You Can See Forever.
He is perhaps best known as a star of musical films, including Annie Get Your Gun (1950), Show Boat (1951), Calamity Jane and Kiss Me, Kate (both 1953), Rose Marie and Seven Brides for Seven Brothers (both 1954), and Kismet (1955). His television credits include a long-running stint on Dallas.
Keel is survived by his wife and their daughter, three children from a previous marriage, 10 grandchildren, and one great-granddaughter.