The U.S. hit the crossbar three times before Dunn slotted home the game-winning goal off a corner kick.
"We knew that they were going to be great," said U.S. coach Kazbek Tambi, of the Mexican team. "Mexico has a great soccer tradition. Their girls play similar to their men in terms of having a very possession oriented and technical game. We knew that. The key for us was to play our game and get the better of them."
Mexico's backline included Californian Maricela Padilla, younger sister of Jesus Padilla, who plays for Chivas Guadalajara, and Carmen Padilla, a member of Mexico's senior national team. The U-17 Tri also featured Charlyn Corral, a member of the senior national team who hit the crossbar from 25 yards out in the first half.
The win also secures a place in the CONCACAF Under-17 Women's Championship final on Sunday against Costa Rica at Marvin Lee Stadium.
Costa Rica will join the United States in New Zealand as two of the three teams to qualify from CONCACAF. Canada and Mexico will have another chance to qualify on Sunday when they face off for third place.
June 24 in Macoya
USA 1 Mexico 0. Goals: Dunn 90.
USA -- Gaul, Dunn, Colohan, Roberts, Brooks, K.Mewis, S.Mewis, Tymrak, Johnson (Nuzzolese, 64), DiMartino, Verloo (Klei, 79).
Mexico -- Santiago, Alvarado, Cardenas, Diaz, Padilla, Garciamendez, Mayor, Franco, Ortegon (Aponte, 68), Corral, Rangel.
Referee: Carol-Anne Chenard (Canada).