The next year, Määttä scored 19 points in seven games to help the JYP U16 squad to a 4–2–2 record in the qualifiers, enough to avoid relegation.

At the end of the season, Määttä returned to JYP's U20 team en route to an eventual league championship.

He ultimately accepted the latter invitation to play major junior in Canada, thus beginning his Ontario Hockey League (OHL) career the following autumn with London.

In the OHL playoffs, Määttä continued his strong play, only being held pointless in just five of London's 19 post-season games.

After the season and leading up to the 2012 NHL entry draft on 22 June, Määttä was ranked as the eighth-best North American skater.

Määttä was outshined by fellow Finnish defencemen Rasmus Ristolainen and Ville Pokka[17] as Finland finished fourth in its pool, sending the team to the relegation round.

Finland easily won its two relegation matches to stay in the top tier, though a seventh-place finish in the ten-nation tournament was nonetheless considered a disappointment.

He rediscovered his game in early February and carried it through the end of the year, pushing London to its second straight first-place finish (50–13–2–3) this time with the second-best offense in the OHL and the number three defence.

[21] Määttä's solid performance during the Pittsburgh Penguins' training camp, buoyed by an injury to defenceman Kris Letang,[22] earned him a spot on the team's NHL roster to begin the 2013–14 season.

[23] He scored the first points of his NHL career on 8 October 2013, assisting on two goals by Jussi Jokinen in the Penguins' 5–2 win over the Carolina Hurricanes.

[24] On 19 October, Määttä then scored his first career NHL goal against Vancouver Canucks goaltender Roberto Luongo.

On 6 March 2014, Määttä scored his first multi-goal game in the NHL with two tallies in the Penguins' 5–3 loss to the San Jose Sharks.

During a press conference on 27 October, it was announced that Määttä would undergo surgery to remove a tumor from his neck that could be a low-grade thyroid cancer.

In Game Two of the Stanley Cup Playoffs against the Washington Capitals, Brooks Orpik delivered an illegal and late hit to Määttä's head.

[40] Määttä recorded 1 goal and 13 assists for 14 points in the 2018–19 season, placing third on the team with 116 blocked shots and 97 hits in 60 regular-season games.

On 15 June 2019, Määttä was traded to the Chicago Blackhawks in exchange for Dominik Kahun and a 2019 fifth-round draft pick.

[46] In a game against Canada, Määttä was injured from a hit by Boone Jenner, suffering a concussion, thus ending his tournament.

Määttä returned to the international scene to play at the 2011 IIHF World U18 Championships, when he served as an assistant captain for Finland's under-18 team.

He was selected by the coaches as one of the Finns' top three players at the tournament, alongside Joel Armia and Miikka Salomäki.

Määttä capped off his strong Olympic performance with Finland's final goal of the tournament, coming in the third period of a 5–0 victory over the United States in the bronze medal game.