As Edinboro's starting quarterback, he broke "every career passing record in the Pennsylvania State Athletic Conference" and was a two-time finalist for the Harlon Hill Trophy, awarded each year to the individual selected as the most valuable player in NCAA Division II.

He was named the Offensive Back of the Year by the North Central Ohio Football Coaches Association, and was also a first team all-NCOFCA choice.

[10] In the Argonauts second, and final, pre-season game of the 2012 season, Harris threw 13 completions in 15 attempts, for 160 yards and two touchdowns.

Harris continued his strong pre-season play in the first preseason game of the 2013 season; completing eight of 11 passing attempts for 115 yards.

During the 2015 preseason, the Argos announced that Ricky Ray (incumbent starting QB) would miss the first six games of the regular season as he recovered from a torn rotator cuff injury sustained in the 2014 season; Thus making Harris the Argos starting quarterback for Week 1.

[11] Trevor Harris started the first 16 games of the CFL season for the Argos as Ricky Ray's injury rehab took longer than expected.

Within the first few hours of free-agency starting on February 9, 2016, the Ottawa Redblacks announced that Harris had signed with the team to a two-year contract.

[13] Harris made his Redblacks debut partway through the opening game of the season after Burris injured his finger while receiving the ball from a shotgun snap.

Harris completed 17 of 19 passing attempts for 292 yards, with three touchdowns and zero interceptions leading the Redblacks to a dramatic overtime victory over the defending Grey Cup champion Edmonton Eskimos.

[14] Following the game, it was announced that Burris would be placed on the 1-game injured list, making Harris the starting quarterback for Week 2.

[26] Harris concluded the 2017 campaign having set new career highs in completions, attempts and passing yards, leading Ottawa to the playoffs for the third consecutive season.

[33] Harris started in the Eskimos' first 12 games of the season before being ruled out for the team's Week 15 match against the Tiger-Cats with an upper body injury.

In 2021, he started the first four games of the season for the Elks before being placed on the six-game injured reserve list with a neck injury on September 16, 2021.

[40] On October 12, 2021, Edmonton Elks head coach Jamie Elizondo announced that Cornelius would once again get the Week 11 start ahead of a healthy Trevor Harris who wasn't performing to expectations.

He also started the Alouettes' lone playoff game against the Tiger-Cats, which they lost 23-12, bringing Montreal's season to an end.

[43] According to TSN sports journalist and reporter Dave Naylor Harris' contract in the 2022 season would have been upwards of $500,000, with a considerable bonus due in the winter offseason.

[47] When Khari Jones was relieved of his duties in early-July, interim head coach Danny Maciocia continued with Harris as the starting quarterback.

[48] Harris started 15 games for the Alouettes in 2022, with an 8–7 record, and led the team to a second place finish in the East Division.

[52] On July 15, 2023, Harris suffered a tibial plateau fracture of his right knee midway through the fourth quarter of the team's Week 6 loss to the Calgary Stampeders.

[55] After recovering from injury and spending the full six games in the injured list, Harris returned to face his former team, the Alouettes, for the first time since leaving as a free agent, but lost to Montreal 27–24.