Fantenberg played six seasons in his native Sweden before signing with HC Sochi of the Kontinental Hockey League on May 4, 2016. He established himself as a top-pair defenseman with 23 points (three goals, 20 assists) in 44 games and represented Sochi in the KHL All-Star Game. Fantenberg was never drafted by an NHL team but signed a one-year contract with the Los Angeles Kings on May 4, 2017.
Fantenberg adapted quickly to North America and made the Kings' opening night roster for the 2017-18 season. He scored his first NHL goal in a 5-2 loss to the Tampa Bay Lightning on Nov. 9, 2017, and split time between Los Angeles and Ontario of the American Hockey League. Fantenberg finished with nine points (two goals, seven assists) in 27 games with the Kings and had one assist in four games during the 2018 Stanley Cup Playoffs.