The U.S. took an early lead, but Reegan O'Gorman responded with a try in the sixth minute to make it 5-3 for Canada.
Dylan Horgan converted a penalty in the 31st, Luke Bradley scored a try in the 38th and Canada took a 13-6 lead into the break before exploding in the second half with 28 unanswered points.
Patrick Finlay and Andrew Coe added tries five minutes apart to make it 25-6 for the Canadian side less than 10 minutes into the second half.
Guiseppe du Toit added back-to-back penalty converts before Finlay scored his second try of the game in the 66th to make it 38-6. Du Toit tacked on one more convert in the 76th.
Including Monday's 24-23 victory, the Canadians took the two-match total point series from the Americans by a combined score of 65-29.
The Junior World Rugby Trophy will be held in Portugal in May. Canada joins already-confirmed sides from Fiji, Georgia, Hong Kong, Namibia, Portugal, Tonga and Uruguay.
Canada has its sights set on bettering last year's seventh place finish.