He was originally signed by the Pittsburgh Steelers of the National Football League (NFL) as an undrafted free agent in 2009.

Reilly has also been a member of the BC Lions of the CFL and the NFL's Pittsburgh Steelers, Green Bay Packers, St. Louis Rams and Seattle Seahawks.

Reilly played three seasons for Kamiakin High School in Kennewick, Washington, before relocating to Kalispell, Montana.

[2] Reilly was a co-runner-up for the Harlon Hill Trophy, which is presented to the nation's top Division II football player, as a senior.

[citation needed] In 2007, he started all 13 games and completed 271 of 435 passes (62.3%) for 3,386 yards, 30 touchdowns and 10 interceptions while be named Second-team All-North Central.

Reilly completed 19 of 28 throwing attempts for 276 yards with two touchdowns and one interception, leading the Lions to a victory over the Edmonton Eskimos 39–19.

[citation needed] On January 31, 2013, Reilly was traded to the Edmonton Eskimos by the BC Lions; the trade included the exchange of the clubs' second round picks in the 2013 CFL Draft and the Lions receiving the Eskimos' second round pick in the 2014 CFL Draft.

[7] Entering the 2013 CFL season, Reilly was in open competition with Matt Nichols for the starting quarterback job.

[citation needed] On November 29, 2015, the Eskimos won the 103rd Grey Cup with a score of 26–20 over the Ottawa RedBlacks in Winnipeg, Manitoba.

After the game, he received the Grey Cup Most Valuable Player award after completing 21-of-35 pass attempts for 269 yards, two touchdowns and no interceptions.

[12] On February 12, 2019, Reilly became a free agent and subsequently signed a four-year contract with the BC Lions later that day.

However, Reilly suffered a wrist injury early in Week 18 against the Eskimos who would win the match and eliminate the Lions from playoff contention.

[15] In November 2020, Reilly filed a grievance against the BC Lions claiming a portion of his salary was guaranteed, even though the 2020 season was cancelled due to the COVID-19 pandemic.

[17] After beginning the 2021 season with lingering injury issues, Reilly finished the year with 12 starts in 13 games played and passed for a league-leading 3,283 yards.