As a sophomore in 2007, Gardner played football for the University of Detroit Jesuit High School and Academy. He missed five games during his sophomore year because of disciplinary issues, but returned for the final four games of the season. Gardner described the problems he experienced in his sophomore year as a turning point: “I had never sat out before, and that hurt a lot. It was a bad feeling, and I did a lot of growing up. I just knew I had to get better.” After the coach of his team left, Gardner’s grades began to decrease. He wanted to transfer to Saint Martin de Porres High School but the school had closed.

As a junior in 2008, Gardner led Inkster to a state title game and was responsible for 48 touchdowns, 26 passing and 22 rushing. He also compiled 3,287 yards of total offense with 1,886 passing yards and 1,401 rushing yards. He also helped lead Inkster’s basketball team to the state semifinals in March 2009.

As a senior in 2009, Gardner again led Inkster to the state finals game. In the second game of the season, Gardner compiled 340 yards passing and 101 yards rushing against East Kentwood High School. The highlight of Gardner’s senior season came in Inkster’s final regular season game against Steubenville High School in Steubenville, Ohio. Inkster needed a win to advance to the playoffs, and Steubenville had a 60-game home winning streak. Steubenville led 29-28 with 1:19 left in the game. With 39 seconds left, Gardner threw a 63-yard touchdown pass for the win. Gardner described the game-winning drive against Steubenville as follows:

After compiling impressive statistics as a junior, Gardner was invited to a number of combines and camps in the Summer of 2009. In June 2009, Gardner participated in the Nike camp in Champaign, Illinois. In July 2009, Gardner participated in the Elite 11 quarterback camp at Soka University in Aliso Viejo, California, as one of the 12 invited quarterbacks. Gardner earned counselor awards for best footwork and work in the classroom at the Elite 11 camp. After watching Gardner perform at the Elite 11, JC Shurburtt wrote for ESPN.com that, “Gardner has also surprised many. His hustle, leadership and athleticism add up to a tremendous maturity that will help him compete early for playing time in Ann Arbor.” Barry Every, national analyst for Rivals.com, ranked Gardner as the top overall quarterback at the Elite 11 camp based on long-term potential and the ability to win football games, and added, “He may not be as big or fast as current Ohio State quarterback Terrelle Pryor, but he is a close second.”

In September 2009, Gardner was ranked as the top quarterback prospect in the United States by Rivals.com. He was ranked as the No. 4 quarterback nationally by Scout.com and the No. 5 quarterback in the ESPNU 150. In November 2009, Allen Trieu of Scout.com said of Gardner:

The Detroit Free Press wrote in September 2009 that Gardner’s “dual-threat abilities make him an ideal candidate to run Rich Rodriguez’s version of the spread offense. Inkster coach Greg Carter expected Gardner to thrive at Michigan: “We definitely have a similar offense. A lot of the things they do may be what made them think he could do well there. We run the spread, have four receivers at times, sometimes three receivers and two backs. So we play similar…. There was nothing we put in that he couldn’t handle.”

In December 2009, Tom Luginbill, national recruiting director of ESPN Scouts Inc., said he believed Gardner would compete for Michigan’s starting quarterback job in 2010. Luginbill said, “There’s no doubt he’s got the physical tools to do so.” When the Detroit Free Press asked Gardner in December 2009 whether he saw himself starting as a freshman, Gardner said, “Of course I see myself doing that.” Gardner played in two games during his freshman season. He completed seven of ten passes for a total of 85 yards and one touchdown. Gardner also rushed for a total of 21 yards on seven rushing attempts, and finished with one rushing touchdown.

Following an injury to Denard Robinson’s throwing arm in the eighth game of the season against Nebraska, Gardner returned to the quarterback position. He received his first start as Michigan’s quarterback against Minnesota on November 3, 2012. Gardner ran for a touchdown and completed 12 of 18 passes for 234 yards and two touchdowns as Michigan defeated the Golden Gophers, 35-13. Four of the touchdown drives were longer than 75 yards.

On October 19, 2013 against Indiana, Gardner broke John Navarre’s Michigan single-game record with 503 passing yards. He also broke Denard Robinson’s Michigan single-game record with 584 yards of total offense. Gardner and Robinson hold nine of the top ten positions on Michigan’s all-time, single-game total yardage list, as reflected in the following chart (Jake Rudock has the other).

After going undrafted in the 2015 NFL Draft, Gardner agreed to a contract with the New England Patriots.

Gardner was released by the Patriots on May 18, 2015.

Gardner was claimed off waivers by the Pittsburgh Steelers on May 19, 2015. The Steelers practiced with Gardner as a quarterback instead of wide receiver. He did not make the team, and was released on August 17, 2015.

Gardner was signed by the CFL’s Saskatchewan Roughriders on June 27, 2018. He was released by the team on August 15, 2018.