John Donnelly Callahan was born on May 27, 1955, in Los Angeles, California. His father was an engineer and his mother was a grammar school teacher. He has two sisters, Mary and Terese, 1 and 2 years younger than himself. Mary is a professor of accounting at Kennesaw State University (Northwest Georgia). Terese is a Berkeley graduate, business woman, and mother. Both sisters are married. John grew up and has spent his entire life in Southern California except for living in Northern California while attending U.C. Berkeley from 1975 to 1977. John worked at JPL for about a decade.