Mantha was rewarded for his outstanding play by being invited to participate in the 2013 CHL Top Prospects Game,[2] and being named to the QMJHL Second All-Star Team.

[3] During the 2013–14 season, Mantha played in 57 games for Val-d'or and led the QMJHL in scoring, finishing with 57 goals and 63 assists.

[8] Mantha scored the game-winning goal with 52 seconds remaining in the third period of game seven to defeat Baie-Comeau Drakkar and win the President's Cup.

[10] Mantha began the 2014–15 season on the injured reserve list, after fracturing his right tibia on September 15, during a game at the Traverse City Prospect Tournament.

[21] On October 6, 2019, Mantha scored four goals in Detroit's 2019–20 home opener against the Dallas Stars, including a natural hat-trick.

[30] He achieved a Gordie Howe hat trick in a 6–5 season-opening overtime win over the Vancouver Canucks at Rogers Arena on October 9.

[33] In 2012, Mantha represented team Canada at the IIHF World U18 Championship, where he won a bronze medal.

[34] In 2014, Mantha represented team Canada at the IIHF World Junior Ice Hockey Championships.

Mantha was the leading-scorer for Canada, recording five goals and six assists in seven games, and was named to the 2014 WJC All-Star Team.

[35] On April 29, 2019, Mantha was named to the Team Canada roster for the 2019 IIHF World Championship held in Slovakia.

[36] He helped Canada progress through to the playoff rounds before losing the final to Finland to finish with the Silver Medal on May 26, 2019.

[37] Mantha finished the tournament, tied for first in scoring alongside Mark Stone, with 14 points in 9 games and was selected as a top 3 player for Canada.