Johnsson made his Elitserien debut for Frölunda HC on 18 December 2012 in a home game against AIK IF. In his second game, on 26 December at Rögle BK, Johnsson scored his first career Elitserien goal, the game-winning goal in the shootout. in December 2012. On 21 April 2014, Johnsson was named the SHL Rookie of the Year, beating out Frölunda teammates Alexander Wennberg and fellow Maple Leafs prospect Tom Nilsson, as well as HV71's Kevin Fiala. During the 2014–15 season, Johnsson led Frölunda in goals with 22, also contributing 13 assists for 35 points from 55 regular season games played. On 4 June 2015, the Maple Leafs signed Johnsson to a three-year, entry-level contract. The following season was another sensational year for Johnsson, in which he finished sixth in league scoring and helped Frölunda capture their fourth championship. Having stated this would be his last season in Sweden and wanting to begin his North American development, Johnsson debuted for the Toronto Marlies—the Maple Leafs' American Hockey League affiliate—shortly afterwards in order to assist their Calder Cup playoff run. However, his stint was cut short, as in his second North American game, Johnsson was hit in the head by an elbow from Albany Devils player Dan Kelly. Kelly was suspended ten games for the play and Johnsson would not play another game that season. , participating in the 2018 AHL All-Star Game During the 2017–18 season, Johnsson was named to the 2018 AHL All-Star Game as a replacement for fellow Maple Leafs prospect Kasperi Kapanen, who was called-up to the NHL. On 13 March 2018, Johnsson was called-up to the Maple Leafs alongside Calle Rosén. While Rosén was sent back down, Johnsson made his NHL debut on 14 March in a 6–5 win over the Dallas Stars. He recorded his first NHL goal in the following game against the Montreal Canadiens, a 4–0 victory. Johnsson recorded his first multi-point game on 2 April 2018 in a 5–2 win over the Buffalo Sabres. Near the conclusion of the 2017–18 regular season, Johnsson was selected for the AHL's Second All-Star team. Johnsson made his NHL playoff debut during the 2018 Stanley Cup playoffs against the Boston Bruins and recorded his first playoff goal on 21 April to help the Leafs win 4–3. After the Leafs were eliminated from the playoffs, Johnsson was sent down to the Marlies to help them in their 2018 Calder Cup playoff run. After leading all players in points during the playoffs, Johnsson was awarded the Jack Butterfield Trophy as MVP of the Calder Cup. As a restricted free agent entering the off-season, Johnsson accepted his qualifying offer from the Maple Leafs, signing a one-year, two-way contract worth $787,500. Following a successful training camp, Johnsson began the 2018–19 season with the Maple Leafs in the NHL. On 24 November, in a game against the Philadelphia Flyers, Johnsson scored his first career NHL hat-trick by scoring three goals in the first period to lead the Leafs to a 6–0 win. On 28 June 2019, Johnsson signed a new four-year, $13.6 million contract with the Maple Leafs worth an average annual value of $3.4 million.
