Halop came from a theatrical family; his mother was a dancer, and his sister, Florence Halop, was an actress who worked on radio and in television.
In 1933, he was given the lead, Bobby Benson, in the popular new radio show The H-Bar-O Rangers, an early credit of Don Knotts as well. From 1934 to 1937, he starred in one of his first radio series, playing Dick Kent, the son of Fred and Lucy Kent, in "Home Sweet Home".
After several years as a radio juvenile, he was cast as Tommy Gordon in the 1935 Broadway production of Sidney Kingsley's Dead End and traveled to Hollywood with the rest of the Dead End Kids when Samuel Goldwyn produced a film version of the play in 1937. Usually called Tommy in the films, he had the recurring role of a gang leader in a series of films that featured the Dead End Kids, later billed Little Tough Guys. 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 was tired of the name "Dead End Kids". He played with James Cagney in Angels with Dirty Faces (1938), and he also played the bully Flashman, speaking with an English accent, in the 1940 film Tom Brown's School Days opposite Cedric Hardwicke and Freddie Bartholomew.
After serving in World War II, he 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 his show business career.
In the 1970s, he played the character Bert Munson, cab driver and close friend to Archie Bunker on the television series "All in the Family", appearing in episodes from 1971-75.
Halop was married at least four times, according to interviews given near the end of his life. Helen Tupper was his first wife from 1946 until their divorce in 1947. On Valentine's Day, 1948, he married Barbara Hoon. Their marriage lasted ten years until their divorce in 1958. His third marriage in 1960 to Suzanne Roe, who had multiple sclerosis, lasted until their divorce in 1967.
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, California. His fourth marriage, to a nurse coworker, whose name has not been publicized, was quickly annulled after she allegedly attacked him. He later moved back in with his third wife, Suzanne, but they chose not to remarry.
Late in life, Halop supplemented his nursing income with film and television roles, including the recurring role of Bert Munson on All in the Family.
Halop 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.