Selected by the Devils in the second round (No. 60) of the 2012 NHL Draft, Severson signed a three-year, entry-level contract Sept. 6, 2012. He played in the Western Hockey League in 2012-13 and led Kelowna defensemen with 42 assists and 52 points. In 2013-14, he scored 61 points (15 goals, 46 assists) and was plus-47 in 64 games.
Severson made his NHL debut Oct. 9, 2014 and finished his first NHL season with 17 points (five goals, 12 assists) in 51 games. With multipoint games against the Florida Panthers (Oct. 11) and the Ottawa Senators (Oct. 25), Severson became the first defenseman in Devils history to have three multipoint games in his first eight NHL games.