The USA outshot Canada 18-8, but the game looked to be headed for a scoreless draw when Nairn, who entered the game in the 81st minute, finished off a wild scramble in front of Canadian keeper Erin McLeod's goal.
Nairn, 18, is the youngest player to score for the USA in more than six years. She was a starter on the U.S. team that won the 2008 Under-20 Women's World Cup last fall and is age-eligible for the 2010 U-20 tournament in Germany. The Maryland product delayed her enrollment at Penn State due to her commitment to the U-20s and will be a freshman for the Nittany Lions in 2009.
"I was first of all excited to get in the game, but [Lauren] Cheney did all the hard work and I was just there to make sure it went in the goal," said Nairn, who debuted for the USA in its 4-0 victory against Canada on May 25. "I'll take it if I can. If Abby [Wambach] didn't make that run and that touch, and Cheney didn't follow it up, I wouldn't have made it. It's awesome. I can't even explain how I feel right now its awesome to even be on this team and be considered for this team so to put one away like that and win the game is a great feeling."
Canada had its chances in the second half but U.S. goalie Hope Solo twice denied Canada star Christine Sinclair with excellent stops.
U.S. coach Pia Sundhage inserted Leslie Osborne into the match in the 62nd minute, giving the Gold Pride midfielder her first cap since recovering from reconstructive ankle and knee surgery in the middle of last year.
"I think this game was very important because of the way we have been playing and working on the attacking third," said Sundhage. "We didn't score that many goals, but we scored a goal. And we gained some confidence by that and we will continue to work on the attack, especially in the attacking third."
July 22 in Charleston, S.C.
USA 1 Canada 0. Goal: Nairn 89.
USA -- Solo, Mitts, LePeilbet, Buehler, Cox, O'Reilly (Osborne, 62), Boxx (Hucles, 46), Lloyd (Nairn, 81), Rapinoe (Heath, 46), Tarpley (Cheney, 46), Wambach.
Canada -- McLeod, Gayle (Stewart, 26), Chapman, Zurrer, Nault (Woeller, 67), Moscato (Lagonia, 80), Matheson, Parker (Robinson, 64), Lang, Filigno (Julien, 73), Sinclair.
Referee: Sandra Serafini (USA).