Callahan was signed as an undrafted free agent out of Division III Wesley College following the 2016 draft. That fall, he played most of the snaps during the preseason at quarterback due to Aaron Rodgers sitting out and Brett Hundley nursing an injury. He made the opening-week 53-man roster as a rookie, but was waived and claimed by the New Orleans Saints a few weeks later. After stops in the Bayou and in Cleveland, Callahan returned to the Packers’ practice squad late that season.
In 2017, Callahan fought with Taysom Hill for the #3 job, again behind Rodgers and Hundley, seemingly losing that battle. However, both he and Hill were released at the end of camp, and this time it was Hill who landed with the Saints on waivers. Callahan returned to the Packers’ practice squad once again before being elevated to the active roster as Hundley’s backup when Rodgers suffered a broken collarbone.
Following the Packers’ trade for DeShone Kizer this offseason, Callahan received a league-minimum salary on an exclusive-rights free agent tender as the team’s fourth quarterback. However, the team’s impending agreement with undrafted rookie Tim Boyle from Eastern Kentucky made Callahan expendable.