Born in Trenton, New Jersey, Hornberger attended the Peddie School in Hightstown, New Jersey. He then graduated from Bowdoin College in Brunswick, Maine, where he was an active member of the Beta Theta Pi Fraternity, and trained at Cornell Medical School before becoming a physician for the U.S. Army during the Korean War. He used his experience at the 8055th Mobile Army Surgical Hospital as background for his work.