He started acting professionally in 1951 in a supporting role in the Broadway stage musical Three Wishes for Jamie, which, while passably successful, toured the West Coast in the summer of the same year.

He then did three successive episodes of Jack Webb's Dragnet and two other television shows before he portrayed Brian "Gadge" Robertson, the bright grandson of a fictive astroscientist in the science fiction B-flick Tobor the Great, 1954.

In between he continued to appear in standard television series such as Waterfront, The Millionaire, Cheyenne, and My Friend Flicka, and various TV theaters, anthologies and dramas.

When Charles Laughton personally cast Billy Chapin for the role of young John Harper in his 1955 film classic The Night of the Hunter, the boy was already considered an "acting technician" among the child performers of his time.

After a private meeting with Billy in his Hollywood home, Laughton told Davis Grubb, the author of the original story: "What I want is a flexible child, and the boy is exactly that.

[8] Chapin's final big screen appearance came just a year after The Night of the Hunter, as young Jody Burrows in the 1956 B-Western Tension at Table Rock, starring Richard Egan.