In 1945 he narrated “The Nazi Plan”, a documentary film using captured propaganda and newsreel footage to dramatize the Nazis rise to power and was used by the prosecution in the International Military Tribunal in Nuremberg. He served as the narrator on various Hollywood films, including House on 92nd Street (1945), Boomerang (1947), and The Iron Curtain (1948).
Throughout his 35-year career in film, Hadley was cast as both a villain and a hero of the law, in such movies as The Baron of Arizona (1950), The Half-Breed (1952), Highway Dragnet (1954) and Big House, U.S.A. (1955), and narrated a number of documentaries. In films, he starred as Zorro in the 1939 serial Zorro's Fighting Legion.
Hadley has a star at 6553 Hollywood Boulevard in the Television section of the Hollywood Walk of Fame. It was dedicated on February 8, 1960.
On December 11, 1974, Hadley died of a heart attack in Los Angeles. He was 63. He was survived by his wife and son.
Hadley was the narrator of several Department of Defense films: Operation Ivy, about the first hydrogen bomb test, Ivy Mike, "Military Participation on Tumbler/Snapper"; "Military Participation on Buster Jangle"; and "Operation Upshot–Knothole" all of which were produced by Lookout Mountain studios. The films were originally intended for internal military use, but have been "sanitized" and de-classified, and are now available to the public.