Kasowitz spent his early years in New Haven, Connecticut, the son of a scrap metal business owner. A childhood friend told The American Lawyer in 1997 that Kasowitz was “a real scrapper and a real hustler” when he played high school basketball, a side of his personality that stayed with him. Later, while working at Rosenman & Colin and playing on its basketball team, Kasowitz wound up in a fistfight with a member of an opposing team. Kasowitz defended himself, claiming that his foe had provoked him with a kick: “I took all appropriate measures to defend myself until my teammates interceded,” Kasowitz said after the incident.