He came from a theatrical family; his mother was a dancer, and his sister Florence Halop was a radio actress. After several years as a well-paid radio juvenile, Billy was cast as Tommy Gordon in the Broadway production of Sidney Kingsley's Dead End in 1935, where he was accorded star status. Traveling to Hollywood with the rest of the Dead End Kids when Samuel Goldwyn produced a film version of the play in 1937, Billy had no trouble lining up important roles, specializing in tough kids, bullies, and reform school inmates in such major pictures as Dust be My Destiny (1939) and Tom Brown's School Days (1940).

A long-standing rivalry between Halop and fellow Dead-Ender Leo Gorcey led to his break with the Dead End Kids and its offspring groups, the East Side Kids and the Bowery Boys.

After serving in World War II, Halop found that he'd 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.

In 1960, he married Suzanne Roe, who had multiple sclerosis. The nursing skills he learned while taking care of his wife led him to steady work as a registered nurse at St. John's Hospital in Malibu. For the rest of his life, Billy Halop supplemented his nursing income with small TV and movie roles, gaining a measure of prominence as Archie Bunker's cab-driving pal Bert Munson on the '70s TV series All in the Family.

Billy was married three times:

Suzanne Roe (17 December 1960 - 1967) (divorced) Barbara Hoon (14 February 1948 - 5 March 1958) (divorced) Helen Tupper (1946 - 14 January 1947) (divorced) He died on November 9, 1976, at the age of 56 from a heart attack and is interred at Mount Sinai Memorial Park Cemetery in Los Angeles, California. The specific location is in the Garden of Sher Mot, Crypt # 64181.