The Denver Broncos selected him in the seventh round with the final pick of the 2017 NFL draft, making him that year's Mr.

After several years as a reserve player on the Broncos and later the Indianapolis Colts, Kelly signed with the Argonauts, gaining success as a starter in the CFL.

[1] He spent his early years in Western New York, his mother's home area, not far from where his uncle Jim was playing professional football.

Chad Kelly was a consistent strong performer in Punt, Pass, and Kick competitions as a youth,[2] becoming national champion four times.

[34] Going into mini-camps for the 2018 season, he would compete with 2016 first-round draft pick Paxton Lynch for the backup quarterback role behind newly acquired Case Keenum.

[35][36] In Week 6 of the 2018 season, Kelly appeared in his first NFL game, kneeling for a one-yard loss at the end of the first half.

[38] He allegedly entered a couple's place of residence after leaving teammate Von Miller’s Halloween party.

[48] After being released from the Colts, Kelly temporarily stepped away from his professional playing career and returned to East Mississippi where he briefly worked as an offensive coach.

[52] The Argos released Antonio Pipkin as part of the team's final roster cuts, which promoted Kelly to the role of backup quarterback to veteran McLeod Bethel-Thompson to begin the 2022 season.

[55][56] In the 109th Grey Cup, after Bethel-Thompson injured his throwing hand in the fourth quarter, Kelly came off the bench with the Argos trailing 23–17 and helped lead them to a 24–23 win against the Winnipeg Blue Bombers.

[59] On August 31, 2023, Kelly and the Argos agreed to a three-year contract extension worth a total of $1.865 million, making him the highest paid player in the league.

On May 7, 2024, the CFL announced that Kelly was suspended for at least nine games after an investigation into allegations made against him and the Argonauts by a former strength and conditioning coach.

[65] In the East Final game against the Montreal Alouettes on November 9, Kelly was involved in a play in which Isaac Adeyemi-Berglund fell on his right leg, resulting in tibia and fibula fractures.

[67] In his absence, Nick Arbuckle started the 111th Grey Cup where the Argonauts defeated the Blue Bombers and Kelly won his second championship.

[68] Kelly's younger brother Casey also played quarterback for St. Joseph's and Mallard Creek High School.

Criminal charges were dropped in exchange for his plea of guilty to disorderly conduct, and he was ordered to complete fifty hours of community service.

[71] In early 2024, Kelly was named in a wrongful dismissal lawsuit by a former Toronto Argonaut coach, with allegations that Kelly had made attempts of "unwanted romantic advances and escalating into instances of threatening language" leading to the coach’s dismissal upon reporting the incidents.