His younger brother Franz Wagner is also an NBA player as well as his teammate, playing small forward for the Orlando Magic.

[4] In 2014–15, he played on Alba Berlin's Bundesliga roster, appearing in four games of the German league, scoring 2.3 points per contest.

[13] On March 19, Wagner's career-high 26 points helped Michigan defeat Louisville 73–69 to advance to the Sweet 16 of the 2017 NCAA Division I men's basketball tournament.

[18][19] On April 10, 2017, both Wagner and teammate D. J. Wilson declared for the 2017 NBA draft, but did not hire agents, which gave them until May 24 to withdraw their names and retain their athletic eligibility to return to Michigan.

[30][31] On March 2, Michigan defeated Nebraska 77–58 in the quarterfinals of the 2018 Big Ten Conference men's basketball tournament.

[32][33][34] The following day, in the semifinals of the Big Ten tournament Wagner scored his 1,000th career point, becoming the 54th Wolverine to reach the milestone.

[40] Wagner, who averaged 12.5 points and 5.5 rebounds in the first four games of the 2018 NCAA basketball tournament,[41] was joined by Wolverines teammates Matthews and Muhammad-Ali Abdur-Rahkman on the West Region All-tournament team.

[50][51] On May 1, Wagner signed with Roc Nation's Joe Branch, who has represented former Michigan teammate Caris LeVert.

[58] Wagner made his G-League debut for the South Bay Lakers on November 3, scoring 17 points in 29 minutes in a 108–106 loss to the Stockton Kings.

[62][63] On March 9, 2019, Wagner scored a season-high 22 points with six rebounds, three assists and a block in his first career start in a 107–120 loss to the Boston Celtics.

[66][non-primary source needed] Wagner also drew three charges in the game and was the first NBA player to ever post 30 points and 15 rebounds (both career highs) with 4 three-point shots off the bench.

[67] Wagner missed 24 games with an ankle sprain before returning to the lineup on February 3 against the Golden State Warriors.

[68] He was named to the Rising Stars Challenge at the 2020 NBA All-Star Weekend,[69] where he scored 16 points for the world team.

[71] On March 26, Wagner made his debut for the Celtics in a 122–114 win over the Milwaukee Bucks, finishing with three points and five rebounds across 10 minutes of play.

[75] On April 28, he debuted for the Magic against the Cleveland Cavaliers, Wagner scored two points, grabbed two rebounds and dished out an assist across nine minutes in a 109–104 win.

Wagner knocked down a 3-pointer that tied the game at 109 with 1 minute and 16 seconds left to play, contributing to the Magic's 112–111 win over the Memphis Grizzlies.

[80] On December 29, 2022, Wagner was suspended by the NBA for two games without pay due to an altercation with Detroit Pistons guard Killian Hayes the day before.

[85] Wagner won gold with the Germany U18 national team at the 2014 FIBA Europe Under-18 Championship Division B in Bulgaria.