Leon was born in New York City to a family of Jewish immigrants from Eastern Europe.
He graduated from Stuyvesant High School and studied Mathematics as an undergraduate at City College of New York.
Afterward, he enrolled as a doctoral student at Columbia University where he studied under mathematician Claude Chevalley, obtaining his PhD in 1953 at the age of 23.
His doctoral thesis was entitled "Theory of Distributions in Locally Compact Spaces".
Ehrenpreis ran the New York City Marathon every year from its inception until 2007.