In 1940 Chapnick joined a recently founded photo agency, Black Star.
During World War II he served in the United States Air Force.
In 1994 he published a book called Truth Needs No Ally: Inside Photojournalism, summarising his many years of experience in the field of dealing with photojournalists.
Chapnick was a principal founder of the W. Eugene Smith competition and Memorial Fund, which awards grants for projects in humanist photography.
After his death in 1996, the Fund established in his memory a grant to encourage and support leadership in fields ancillary to photojournalism.