Frankel began his career as an actor, making a name for himself Off-Broadway in the 1950s and 1960s. By the mid-'50s he was offering writing, acting, and directing classes. He won OBIE awards for his direction of productions of Volpone (in 1957) and Machinal (in 1960).
In 1961, he directed Genet's drama The Blacks at the St. Marks Playhouse with a cast that included James Earl Jones, Cicely Tyson, Roscoe Lee Browne, Louis Gossett Jr., and Maya Angelou. The production ran for more than three years, toured Europe, and won Frankel his third OBIE. Other Off-Broadway shows followed, and Frankel also directed seven Broadway productions between 1961 and 1975, including Once There Was a Russian, A Cry of Players, Indians, The Engagement Baby, The Lincoln Mask, and The Night That Made America Famous.
Frankel continued to be active with his acting workshop until the end of his life; over the course of his career, he taught such performers as Loretta Swit, Judd Hirsch, Lee Marvin, Rod Steiger, and Morgan Freeman. A theater on Bond Street that is named after Frankel opened in 1988. He is survived by a daughter, Laura Ann Frankel, of Laurel, Maryland.