He played college soccer at the University of North Carolina at Chapel Hill, where he was named to the ACC's All-Freshman Team in 2005, was named to the ACC Academic Honor Roll in his freshman, sophomore and junior years, and earned All-ACC and All-American honors in 2008.

During his college years Callahan also played four seasons in the USL Premier Development League with the Raleigh Elite team, which later became the Cary RailHawks U23s.

He made his professional debut on April 25, 2009, as a second-half substitute in Austin's game against Cleveland City Stars.

[1] Callahan set an infamous USL First Division record by getting a red card, and subsequent ejection, 53 seconds into a game for a dangerous slide tackle.

After two seasons with Austin, Callahan signed with Richmond Kickers of the USL Pro league on March 29, 2011.