Magnus Pääjärvi was born in Sweden while his father Gunnar was the coach and manager of IK Vita Hästen.

During the 2005–06 season, as a 14-year-old, he made his debut for Malmö Redhawks in the J20 SuperElit,[2] and scored eight goals in eight games during TV-pucken, leding Skåne to a silver medal.

In 2007, Pääjärvi signed with Timrå IK, where his older brother, Björn Svensson, played for the senior team.

[6][7] On 23 September, in his first pre-season game with Edmonton, Pääjärvi scored a hat-trick and registered an assist for four points as the Oilers defeated the Tampa Bay Lightning, 5–2.

[9] On 10 July 2013, Pääjärvi was traded, along with a second-round pick, to the St. Louis Blues in exchange for left winger David Perron.

[11] On 29 December 2014, the Blues placed Pääjärvi on waivers and, upon clearing, was assigned to St. Louis' AHL affiliate, the Chicago Wolves, where he remained until the end of the 2014–15 season.

The St. Louis Blues made him a qualifying offer to retain his NHL rights and, on 5 July 2015, Pääjärvi filed for Salary Arbitration.

As a free agent leading into the 2019–20 season, Pääjärvi halted his NHL career to sign a two-year contract with Russian club, Lokomotiv Yaroslavl of the KHL, on 22 October 2019.

[18] In the midst of his final year under contract in the 2020–21 season, Pääjärvi added 3 goals and 11 points in 26 appearances before he was traded to HC Dynamo Moscow in exchange for Teemu Pulkkinen on 30 November 2020.