Josefson made his Elitserien debut on 28 February 2008, against Timrå IK, and became a regular member of Djurgården's senior team.

[3] Josefson attended high school at Vittra Gymnasium in Sweden while playing for Djurgården.

[2][18] At the end of the season, Josefson had scored 5 goals and 16 points, a record for players 18 or younger playing with the senior team.

In the second game, Josefson scored a hat trick and added two assists against the Malmö Redhawks.

[26] Djurgården lost the game 7–6, but on 20 October, Josefson would get his revenge when he scored the game-winning goal in the 3–2 victory over HV71 20.

[27] Josefson missed six Elitserien games due to the World Junior Hockey Championship, but was back in the roster against Timrå IK on 9 January.

[36] Josefson was called up to the Devils on 14 October 2010, and made his NHL debut the next day, in a 3–2 victory against Colorado Avalanche at the Prudential Center.

[40] At last, Josefson returned from his injury on 7 January 2011, when the Albany Devils played against the Charlotte Checkers.

[42] The Devils failed to reach the playoffs and Josefson ended up with three goals and ten points in 28 regular season games.

[44] On 3 April 2012 in a game against the Islanders, Josefson bumped awkwardly into Jay Pandolfo resulting in a broken left wrist.

After seven seasons within the Devils organization, Josefson left as a free agent and signed a one-year, $700,000 contract with the Buffalo Sabres on 1 July 2017.

[46] In the 2017–18 season, Josefson was limited to just 39 games through various injuries to contribute with just 2 goals and 4 points for the bottom placed Sabres.

Having played 8 successive seasons in the NHL amassing 315 games, Josefson as an impending free agent from the Sabres opted to return to his original club in Sweden, Djurgårdens IF, agreeing to a multi-year contract through 2022 on 27 April 2018.

[51] He left the team in the morning before the fourth game due to an accident in which his father and uncle had been exposed to carbon monoxide poisoning.

[54] Josefson participated in the World Junior Championship for the second time when he was named for Team Sweden by head coach Pär Mårts for the 2010 World Junior Championships, along with fellow Djurgården teammates Daniel Brodin and Marcus Krüger.