On April 28, 2018, Allen signed with the Carolina Panthers as an undrafted free agent. He was waived on September 1, 2018, and was signed to the Panthers practice squad the next day. He was released on September 10, 2018. He was re-signed to the Panthers practice squad on October 30, 2018. On December 20, 2018, Allen was promoted to the active roster to back up Taylor Heinicke after starter Cam Newton was shut down for the season. On December 23, 2018, he made his NFL debut against the Atlanta Falcons in relief for an injured Heinicke going 4-of-4 with 38 yards in his two drives before Heinicke came back into the game at the two minute warning of the first half. Allen would go on to make his first NFL start in the last game of the season against the New Orleans Saints. He went 16-of-27 with 228 passing yards, with two passing touchdowns and one rushing touchdown. He suffered an injury early in the fourth quarter and was relieved by Garrett Gilbert.
