Templeton was born in New York City and graduated from Yale University, where he was a member of Elihu senior society, as was his father.

In 1977, he went to work at the Children's Hospital of Philadelphia as a pediatric surgeon and trauma program director.

[1] He retired in 1995, as chief of pediatric surgery at Children's Hospital of Philadelphia to join the John Templeton Foundation and took over the leadership when his father died in 2008.

[6] In a Philadelphia Inquirer article it was asserted that the Templetons had, between John and his wife, Josephine, donated $1 million to parties opposing same-sex marriage.

Survivors include his wife, Josephine (Pina) Gargiulo Templeton, whom he married in 1970; two daughters, Heather Dill and Jennifer Simpson; a brother; and six grandchildren.