The Los Angeles Kings selected Wagner, a forward from Calgary, in the fourth round (No. 99) of the 2015 NHL Draft after he scored 20 goals in his second full season with Regina of the Western Hockey League. He remained with Regina for the next two seasons, scoring 28 goals in 2015-16 and 30 in his final junior season.
The Kings signed Wagner on March 6, 2017, and after offseason shoulder surgery joined Ontario of the American Hockey League, scoring 10 goals in 50 games and impressing with his speed and work ethic. He made the Kings roster to start of the 2018-19 season, made his NHL debut against the San Jose Sharks on Oct. 5, 2018, and scored his first goal against the Colorado Avalanche on Nov. 21, 2018.