His most notable Broadway production was Arthur Kopit's Indians starring Stacy Keach, who was nominated for the 1970 Tony Award as Best Actor for his portrayal of Buffalo Bill.
His other Broadway productions included A Cry of Players (1968), Kurt Weill's Lost in the Stars (1972) and Harry Chapin's The Night That Made America Famous (1975).
On August 4, 1973, his Mercer Arts Center, a complex of seven small theaters, which had been located on the first two floors of the residential Broadway Central Hotel, physically collapsed.
Frankel, who had been conducting a rehearsal at the time, noticed the ceiling and walls beginning to buckle and heroically led the actors and several residents to safety; five people died in the collapse.
His son, an aspiring actor, who studied at his father's school had struggled with psychiatric illness which led him to leap off the top of a Manhattan 17-floor-building in 1995 during a psychosis from which he survived.