Fantenberg was a product at the youth level of IF Troja-Ljungby. In 2009, he moved to HV71, where he made his debut in the Elitserien during the 2010–11 Elitserien season. After five years with the club, he signed with Frölunda HC in 2014. In his second year with Frölunda, the 2015–16 campaign, he helped win the Swedish championship and the title in the Champions Hockey League. Fantenberg secured his first contract abroad prior to the 2016–17 season when he inked a one-year contract with HC Sochi of the Kontinental Hockey League. Fantenberg adapted quickly to the KHL, embracing a top-pairing role in appearing in 44 games. In contributing with 3 goals and 23 points, he was Sochi's initial representative to be selected for the KHL All-Star Game. A free agent following his successful season in Sochi, Fantenberg gained NHL attention and agreed to a one-year, entry-level contract with the Los Angeles Kings on 4 May 2017. Fantenberg participated in the Kings' rookie and main roster training camp before making his pre-season NHL debut with the Kings on 16 September 2017. In quickly adapting to the North American style, Fantenberg made the Kings opening night roster for the 2017–18 season. He made his NHL debut with the Kings, playing 17 minutes in a second-pairing role in a 2–0 victory over the Philadelphia Flyers on 5 October 2017. On 9 November 2017, he scored his first career NHL goal, coming in a 5–2 loss against the Tampa Bay Lightning. Fantenberg made the Kings' opening roster to begin the 2018–19 season. In his first full season in the NHL, he earned a third-pairing role on the blueline and appeared in 46 games with the Kings, registering two goals and one assist. On 25 February 2019, he was traded to the Calgary Flames in exchange for a fourth-round pick in the 2020 NHL Entry Draft. On 1 July 2019, Fantenberg left the Flames as a free agent and signed a one-year $850,000 contract with the Vancouver Canucks. Fantenberg made the Canucks roster out of training camp as the team's seventh defenceman, but was unable to draw into the lineup. Fantenberg finally made his season debut on December 1, before scoring his first goal with Vancouver two nights later, in a 5–2 win over the Ottawa Senators.
