During the 2017-18 season, Jonsson-Fjallby recorded 16 points (7g, 9a) in 42 regular season games and eight points (6g, 2a) in 11 playoff games with Djurgårdens IF. The Stockholm, Sweden, native also led Sweden to a silver medal in the 2018 U-20 World Junior Championship, earning four points (2g, 2a) in seven games.
The Capitals selected Jonsson-Fjallby in the fifth round, 147th-overall, in the 2016 NHL Entry Draft. Jonsson-Fjallby was ranked 92nd overall among international skaters by NHL Central Scouting. The 6'0", 170-pound forward made his SHL debut in 2016-17 and recorded one assist in 24 games with Djurgårdens IF. Jonsson-Fjallby also registered 37 points (17g, 20a) in 32 games with Djurgårdens IF's U-20 affiliate in the 2016-17 season. Jonsson-Fjallby represented Sweden in seven games between Sweden's U19 and U20 teams and recorded one goal during the 2016-17 season.
Prior to the 2016 NHL Draft, Jonsson-Fjallby registered 29 points (13 goals, 16 assists) in 39 games with Djurgarden (Sweden-Jr.) in 2015-16. Additionally, he earned four points (two goals, two assists) in seven games with Sweden at the 2016 U18 World Championship. Jonsson-Fjallby is one of six Swedish players drafted by Washington since the 2012 NHL Draft.
