The goal was Wambach's first international goal since she broke her left leg in the USA's final tune up for the 2008 Olympics.
Substitute Lauren Cheney, who replaced Wambach on the Olympic roster, set up Wambach for the game's only goal, slipping a perfect ball behind Canadian defender Candace Chapman that Wambach ran on to and after a touch rolled her past Canadian goalkeeper Karina LeBlanc.
"I think it's pretty fitting that Lauren Cheney, the girl who replaced me going to the Olympics after my leg breaking, played me an amazing ball," said Wambach. "I took a great first touch, and just hit it far post. Thankfully, it didn't go wide."
The USA dominated Canada, 21-3, but could put only one goal past LeBlanc, the Los Angeles Sol goalie.
In the eighth minute, midfielder Angela Hucles rocked the crossbar from 19 yards out, but her shot struck the underside of the bar and bounced away.
The game marked the return of Cat Whitehill. who earned her first cap since she tore her ACL in June 2008. Whitehilll played very well in her 45 minutes before giving way to Mitts at halftime.
Captain Christie Rampone missed the game with an emergency abdominal surgery.
July 19 in Rochester, N.Y.
USA 1 Canada 0. Goals: Wambach 78.
USA -- Barnhart, Whitehill (Mitts, 46), LePeilbet, Buehler, Chalupny, O'Reilly (Heath, 75), Hucles, Boxx, Rapinoe, Rodriguez (Cheney, 63), Wambach.
Canada -- LeBlanc, Chapman, Zurrer, Wilkinson (Gayle, 89), Timko (Nault, 58), Matheson, Parker (Moscato, 85), Robinson (Stewart, 46), Lang (Pacheco, 80), Filigno (Kyle, 75), Sinclair.
Referee: Margaret Domka (USA)