Callahan played college football for the UCLA Bruins and began his coaching career with the Denver Broncos in 2010 and served as an assistant coach for the Cincinnati Bengals, Detroit Lions, and Oakland Raiders.

Born in Champaign, Illinois, Callahan attended De La Salle High School in Concord, California, before attending the University of California, Los Angeles (UCLA), where he played for the university football team as a quarterback and holder from 2002 to 2005.

Callahan then served as the offensive coordinator and quarterbacks coach at Junípero Serra High School in San Mateo, California from 2008 to 2009.

Callahan was part of the Broncos staff when the team won Super Bowl 50 over the Carolina Panthers.

[5] During his time there, the Bengals made it to Super Bowl LVI but lost to the Los Angeles Rams 23–20.