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