Adams graduated from Bishop Alemany High School in Mission Hills, Los Angeles, in 2011.

He would eventually receive only two scholarships offers from Football Championship Subdivision schools Portland State and Eastern Washington.

[3] After redshirting the 2011 season as Eastern Washington's Offensive Scout Team Player of the Year,[1] Adams would split time at quarterback with Junior Kyle Padron in 2012.

[5] He was also one of 20 players on the ballot for the Jerry Rice Award, given to the top freshman in FCS by The Sports Network, which he finished in sixth place.

[12] Adams broke two bones in his foot during a win over Idaho State on October 4, causing him to miss four games.

[16] Adams was again a finalist for the Walter Payton Award, but was the runner-up for the second-straight year, losing to John Robertson of Villanova.

[17] In January, Adams asked for, and was granted, a release from his scholarship from Eastern Washington so that he could speak to other programs in regards to transferring for his senior season.

[2] On February 9, 2015, Adams announced his plans to accept a scholarship to Oregon and transfer for his senior season, choosing to play for the Ducks over remaining at Eastern Washington.

However, later that summer it was reported that Adams needed to complete a math course in order to graduate from Eastern Washington University.

On August 13, 2015, he passed the math class to graduate from Eastern Washington and officially joined the Oregon Ducks.

[26] On May 20, it was announced that Adams' rights were traded to the Montreal Alouettes in exchange for a 1st-round pick in the 2017 CFL Draft.

[39] Adams missed the rest of the year, but was signed a two-year contract extension with the Alouettes on October 28, 2018, alongside several other quarterbacks.

However, after Pipkin suffered a leg injury early in the season, Adams was announced the starting quarterback.

[42] Adams returned from injury in Week 10 and led the Alouettes to a come-from-behind victory over the defending champion Calgary Stampeders.

[43] Adams continued his stellar play late into the season, leading the Alouettes to multiple come-from-behind victories, propelling his team to second place in the East Division with a record of 7–5.

With a playoff spot locked up several weeks before the season ended, Adams was named the team's Most Outstanding Player.

[46] Adams started his first CFL playoff game, the East Semi-Final against the Edmonton Eskimos, who qualified for the crossover.

Adams suffered a shoulder injury in the last few minutes of the team's Week 10 match against the Ottawa Redblacks.

[49] On November 10, 2021, head coach Khari Jones ruled out the possibility of Adams returning to the lineup late in the season.

[52] After the first four games of the season, Khari Jones was fired and replaced by general manager Danny Maciocia, who retained Harris in his role as the starting quarterback.

[54] One week later, Adams was placed on the six-game injured reserve list with tendinitis in his throwing elbow.

[55] On August 31, 2022, Adams was traded to the BC Lions, whose starting quarterback, Nathan Rourke, had suffered a Lisfranc injury, in exchange for a first-round pick in the 2023 CFL Draft.

[59] Rourke signed with the Jacksonville Jaguars in January 2023,[60] which created an opening for the starting quarterback position for the Lions.

[66] He dressed for 18 games, starting in 16, where he completed 333 of 488 pass attempts for 4,769 yards with 31 touchdowns and 18 interceptions, which were all career-high totals.

The highlight of the game for Adams was a 45-yard Hail Mary touchdown pass to Justin McInnis on the final play of the first half.

Vernon will not be traded and is too good a player, too valuable a leader and a huge part of our team.”[71] Rourke started the Lions' next eight games.