Halop came from a theatrical family: his mother was a dancer, and his sister Florence Halop was a radio actress. After several years as a radio juvenile, Billy was cast as "Tommy Gordon" in the Broadway production of Sidney Kingsley's Dead End in 1935, and traveled to Hollywood with the rest of the Dead End Kids when Samuel Goldwyn produced a film version of the play in 1937. In an interview in his later years, he claimed that he was paid more than the other 'Dead End' actors, which had contributed to bad feelings in the group, and that he hated the name 'Dead End Kids'. He also played the vicious bully Flashman in the 1940 Tom Brown's School Days opposite Cedric Hardwicke and Freddie Bartholomew.
After serving in World War II, Halop found that he had grown too old to be effective in the roles that had brought him fame. At one point, he was reduced to starring in a cheap East Side Kids imitation at PRC studios, Gas House Kids (1946). Diminishing film work, marital difficulties, and a drinking problem eventually ate away at Halop's show business career.
Halop married seven times. His first wife, from 1946 until their divorce on January 14, 1947, was Helen Tupper. On Valentine's Day, 1948, he married Barbara Hoon. Their marriage lasted ten years, until their divorce on March 5, 1958. Although the first three of Halops marriages are well documented, there is evidence that there were four other very brief marriages that had been annulled. It is also documented that Halop had a heavy affair with actress Judy Garland.
His marriage, on December 17, 1960 to Suzanne Roe, who had multiple sclerosis, lasted until their divorce in 1967. However, the nursing skills he learned while taking care of his third wife led him to steady work as a registered nurse at St. John's Hospital in Santa Monica. For the rest of his life, Billy Halop supplemented his nursing income with small TV and movie roles, gaining a small measure of prominence as Archie Bunker's cab-driving pal Bert Munson on the '70s TV series All in the Family.