According to Etymonline the expression is from the early ‘90, but they add no details:
Reality television from 1991.
Reality television as a genre appears to date back at least to the ‘40s as suggested by Wikipedia :
Television formats portraying ordinary people in unscripted situations are almost as old as the television medium itself. Producer-host Allen Funt's Candid Camera, in which unsuspecting people were confronted with funny, unusual situations and filmed with hidden cameras, first aired in 1948, and is often seen as a prototype of reality television programming.
but it is not clear when the term reality was first used to refer to a TV show and which the show in question was.
When and for what show was “reality” first used for a TV program?
Was it a BrE or an AmE usage originally?