Callahan has guided 14 offensive linemen to a total of 35 Pro Bowl selections in 27 years in the NFL, including 24 seasons coaching offensive line. He was selected as the PFWA Paul "Dr. Z" Zimmerman Award recipient in 2024, honoring lifetime achievement as an assistant coach in the NFL. In Callahan's 48-year coaching career, he has served as a head coach at both the collegiate and NFL levels.

Callahan spent the past two seasons (2024-25) as the offensive line coach for the Tennessee Titans following four seasons (2020-23) in the same role with the Cleveland Browns. In his first season in Cleveland, Callahan coached three offensive linemen to All-Pro honors, with tackle Jack Conklin earning first-team All-Pro accolades in his first season (2020) and guards Wyatt Teller and Joel Bitonio earning second-team recognition. Bitonio was named first-team All-Pro in both of Callahan's next two seasons as well (2021-22) and was a Pro Bowl selection throughout all four years of Callahan's tenure. Teller also earned a second consecutive second-team All-Pro selection in 2021 and was named to the Pro Bowl in each of Callahan's next three seasons (2021-24). 

Prior to Cleveland, Callahan spent five seasons with Washington, starting as offensive line coach (2015-16) before being promoted to assistant head coach/offensive line (2017-19) and interim head coach in 2019.

He also spent three seasons (2012-14) as offensive coordinator and offensive line coach for the Dallas Cowboys following four seasons (2008-11) as assistant head coach and offensive line coach for the New York Jets. Callahan also spent time in collegiate athletics, serving as the head coach for Nebraska from 2004-07.

Callahan served as the head coach of the Oakland Raiders from 2002-03 after starting with the organization as offensive coordinator and tight ends coach (1998) before being promoted to offensive coordinator and offensive line coach (1999-01) and finally taking over as head coach. Prior to Oakland, Callahan entered the NFL coaching ranks as the offensive line coach for the Philadelphia Eagles (1995-97). Callahan also spent time in various roles at Illinois, starting as a graduate assistant (1980-81) before being promoted to special teams and tight ends coach (1982-83), offensive line coach (1984-85) and quarterbacks coach (1986). He also served as offensive line coach at Northern Arizona (1987-88), offensive coordinator at Southern Illinois (1989) before serving as offensive line coach at Wisconsin (1990-94.