A frequent guest on the "The Ed Sullivan Show" (1948).
Collapsed just after coming off-stage at a performance at the Rainbow Room at New York's Rockefeller Center. Died in hospital early the next day.
One of Frank Sinatra's favorite comedians, Jack double-billed occasionally with Frank Sinatra Jr..
The abrasive comedian often opened his act with the line, "Good evening, opponents!"
Wife Monnet Alvarez was an artist and his daughter, Brenda Leonard Cowart, is an accomplished jazz pianist who has played for such Las Vegas shows as "City Lites", "Folies Bergere" and "Lido de Paris".