He was signed by the Green Bay Packers as an undrafted free agent in 2016, and was briefly a member of the New Orleans Saints, Cleveland Browns, Philadelphia Eagles, Tampa Bay Buccaneers, Baltimore Ravens, Detroit Lions, and Seattle Dragons.

After graduation from Holy Spirit High School in his hometown of Absecon, New Jersey,[1][2] Callahan appeared in four games for Wesley as a true freshman in 2011.

[4] In the following week, Callahan led the Wolverines to victory with a 43-yard Hail Mary touchdown on fourth and 21 with under a minute remaining to beat archrival Salisbury.

Behind 31–0 after the first quarter, the Wolverines fell short by just three, 62–59; Mount Union recovered an unsuccessful onside kick with ninety seconds remaining.

[8] As a redshirt junior in 2014, Callahan threw for 4,190 yards and 46 touchdowns with 10 interceptions, making it the best season ever for a Wesley quarterback.

In the fourth game, Wesley beat Southern Virginia 47–7, with Callahan passing for 339 yards and five touchdowns.

[10] Callahan and the Wolverines dominated in their first three playoff games, outscoring opponents 152–20 until losing again to Mount Union, this time by a wider margin, 70–21.

Returning for a fifth year in 2015, Callahan threw for 5,063 yards and 55 touchdowns and led Wesley back to the D-III playoffs.

Not selected in the 2016 NFL draft, Callahan signed as a free agent with the Green Bay Packers on May 6.

[17] Callahan was claimed off waivers by the Cleveland Browns on October 24,[18] and released by the team on November 28.