Clemenson was born and raised in Humboldt, Iowa, the son of drug store owners Ruth Alzora (Dayton) and Ernest Arnold Clemenson. In his early teens, he delivered the Des Moines Register, which had a long tradition of awarding scholarships to top east-coast preparatory schools to a select few "paper boys." Clemenson, a straight-A junior high school student, won a Register scholarship to Phillips Academy in Andover, Massachusetts. In 1973, while Clemenson was enrolled at Phillips Academy, his father died at age 58. He participated in many theatrical productions at Phillips Academy.