Stone scored one marker in the first period and added two more in the second, while Mantha buried his two goals less than two minutes apart in the third.
Mantha went between the legs on his first one, making a bid for goal of the tournament.
Yasin Ehliz produced the lone goal for the losing side.
Canada handed Germany their first loss of the tournament and improved to 4-0-0-1.
Next up for Canada is a matchup with Denmark on Monday, and then a clash with the United States on Tuesday.
Germany will face the U.S. on Sunday, followed by Finland on Tuesday.