Nichols was a four-year starter at Eastern Washington, where he compiled 996 completions on 1608 attempts (62%) for 12,616 yards, 96 touchdowns and 46 interceptions in 47 games (42 starts). Nichols' career passing yards are the most in Big Sky Conference history and ranks sixth overall in NCAA history. Nichols also holds the school record for touchdown passes with 96 and was named the 2007 and 2009 Big Sky Conference Offensive Player of the Year. The Business Management major attended college with former Winnipeg Blue Bomber defensive end Greg Peach.
Nichols attended training camp with the Dallas Cowboys as an undrafted free agent in 2010 and appeared in two pre-season games, recording 10 completions in 16 attempts for 81 yards and two interceptions. He was released on August 19.
Nichols was signed as a free agent by the Edmonton Eskimos of the Canadian Football League on October 13, 2010. Nichols spent the next three games on the practice roster before being released on November 2. On February 4, 2011, he was re-signed by Edmonton through the end of the 2012 season. In the playoffs of the 2012 CFL season Nichols sustained a devastating broken leg injury. He worked hard all off-season in order to be able to play for the 2013 CFL season. Entering the 2013 CFL season Nichols was in open competition with Mike Reilly for the starting quarterback job. Nicols was awarded the start for the first preseason game on June 14, 2013. However, he left the game early on after attempting to tackle a player after throwing an interception. The tests revealed that he had torn ACL and required surgery; this caused him to miss the entire 2013 season. In the offseason, Nichols was signed to a two-year contract extension through the 2015 CFL season.
On September 2, 2015, Nichols was traded to the Winnipeg Blue Bombers. Nichols played in numerous games for the Bombers in 2015, due in large part to various injuries to starting quarterback Drew Willy. Nichols started the 2016 season as the backup to Drew Willy, however following a 1–4 start to the season head-coach Mike O'Shea announced that Nichols would be the starting quarterback for their week 6 match-up against Edmonton. In 2016, Nichols began his tenure as the Bombers starting quarterback with 7 wins in a row propelling the Bombers to a record of 8–4. The Bombers would finish the season with a record of 11–7, finishing 3rd place in the Western Division. In the playoffs the Bombers were unable to hold onto a 19-point lead over the BC Lions, falling 32–31 thus ending their 2016 campaign. Drew Willy was traded in the middle of the season, and veteran quarterback Kevin Glenn was released in January 2017, confirming the Bombers belief that he could be their starting quarterback for the foreseeable future. This sentiment was reinforced when they signed him to a 3-year contract extension on January 18, 2017. Nichols had a breakout 2017 season, setting career highs in every major category (attempts, completions, yards, touchdowns). Nichols led the Bombers to a 12-6 record, clinching a home playoff game; however, the team was defeated by the Eskimos in the first round.
During the lead up to the start of the 2018 season Nichols suffered a non-contact knee injury in practice, and it was announced he would miss four to six weeks, returning in four as he made his 2018 debut on July 7 against the B.C. Lions. Nichols had a strong season for the Blue Bombers, leading the team to a record of 10-8 and a playoff birth. The Bombers advanced to the West division finals after defeating the Saskatchewan Roughriders 23-18 on the road, but we unable to overcome the Calgary Stampeders who went on to win the 106th Grey Cup. Nichols was having an outstanding 2019 season and was leading the league in touchdown passes (15) and passer rating (107.2), but suffered an upper-body injury in Week 10 which caused him to be placed on the six-game injured reserve.