Koepp grew up in the small town of Pewaukee, Wisconsin. He went to a variety of colleges over a leisurely-paced academic career, and recounts his junior year as being "three of the happiest years of my life." He cites the University of Wisconsin at Madison and the film school at UCLA as particular highlights.

He credits much of his success as a writer to the Sisters of the Third Order of Saint Francis at Saint Anthony's Church and School, because they told him early on his handwriting was so awful he'd better learn to type immediately.