U.S. forward Marcia Wallis replaced Kim Patrick to open the second half, and seven minutes later she beat Mexican defender Barbara Almaraz at the top of the box and hit a right-footed shot from 19 yards that gave Mexican goalkeeper Sandland Quinones no chance.
Wallis also scored the game's first goal in the United States' 6-0 victory against Costa Rica on Friday. That goal came just two minutes into the game.
But this time Wallis' goal was not enough and, in the 89th minute, Leyva's equalizer broke the United States' Pan American shutout streak of 179 minutes.
Leyva gathered the ball outside the box after U.S. defender Karyn Hall used a sliding tackle to clear the ball from the penalty area. Leyva uncorked a wicked left-footed drive from 16 yards that beat U.S. goalkeeper Lauren Whitt to the near post.
"We played better today than we did against Costa Rica," U.S. midfielder Becky Myers said. "But the Mexican team played real well and we just didn't take advantage of our chances."
Mexico almost jumped out to an early lead in the game's first twenty minutes when Lauren Whitt did well to save Leyva's free kick from 22 yards.
Throughout the first half the U.S. was unable to find the form that netted the team three goals in the first seven minutes of Friday's game. Although the United States did launch nine shots in the first half, only two posed legitimate threats to the Mexican team.
Patrick's blast from 30 yards was stopped by Quinones and Quinones made a diving save on Lauren Molinaro's header in the 42nd minute.
U.S. coach Jay Hoffman chose to shufle his starting line-up, making five adjustments from the team that started Friday, including a goalkeeper change. Whitt replaced Hope Solo in the nets and the pair will likely continue to rotate throughout the tournament.
With the four points, the United States stands atop the women's tournament, with Canada (three points) yet to play its second game. The match was Mexico first of the tournament.
The United States squad will not play again until July 29, when it faces Trinidad & Tobago at 6:00 p.m. at Red River College. Trinidad & Tobago lost its opening game to Canada 7-1 on July 23.