He wrote for Sid Caesar in the 1950s, along with Woody Allen. When Allen later cast Harold's grandson Timothée Chalamet in his film A Rainy Day in New York (2019), he was initially unaware of the family relation.
His parents, Sadie (Kasner) and Jacob Flender, were Austrian Jewish immigrants.
Married to Enid S. (Rodman), who was born in New York, to Russian Jewish parents.