Riordan was raised in San Antonio, Texas.[4] He graduated from Alamo Heights High School in 1982 and the University of Texas at Austin in 1986, where he double-majored in English and history.[5] Riordan taught English and history, including Greek mythology, in middle schools in the San Francisco Bay Area and in San Antonio.[6] He was awarded St. Mary's Hall's first Master Teacher Award in 2002.[7]