Cornyn was born in Houston, the son of Atholene Gale Cornyn (née Danley) and John Cornyn II, a colonel in the U.S. Air Force. He attended the American School in Japan after his family moved to Tokyo in 1968, and graduated from it in 1969. In 1973, he graduated from Trinity University, where he majored in journalism and was a member of Chi Delta Tau. Cornyn earned a Juris Doctor from St. Mary's University School of Law in 1977 and an LL.M. from the University of Virginia School of Law in 1995. He was named the St. Mary's Distinguished Law School Graduate in 1994 and a Trinity University Distinguished Alumnus in 2001.