The goal gave Leroux, winner of the Golden Ball and Golden Shoe at the 2008 Under-20 Women's World Cup, a tournament-high six goals.
Leroux teamed up with Kristie Mewis to break the scoreless deadlock when she first-timed a long pass back into the center circle to Mewis, who slid a perfect ball on the ground in the path of the sprinting Leroux, who outran a Mexican defender, took one hard touch into the left side of the box and beat Mexico goalie Aurora Santiago.
Leroux's goal came shortly after Mexico star Veronica Corral had two excellent chances to put the Tri ahead.
Mexico had an 11-10 edge in shots, marking the first time the Americans were outshot in five games.
Costa Rica's win over Canada meant Canada failed to qualify for a FIFA Women's World Cup at the youth level for the first time.
Canada won the 2008 Concacaf U-20 title.
Jan. 30 in Guatemala City
USA 1 Mexico 0.
USA - Leroux (K.Mewis) 87th minute.
USA -- Henninger, Quon (S.Mewis , 83), Dunn, Pressley, Johnson (DiMartino, 56), Short (Noyola, 44), Bywaters, Brooks, Nairn, K.Mewis, Leroux.
Mexico -- Santiago, Sierra, Garciamendez, Diaz, Miranda, Rangel, Godoy, Alvarado, Kotero (Figueroa, 70), Mayor, Corral.
Referee: Yesli Sarai Rivas (El Salvador).