Kemar Lawrence, one of nine current or former MLS players in the starting lineup, broke a scoreless deadlock when he scored on a bending free kick, leaving El Tri keeper Jesus Corona motionless, in the 88th minute.
Jamaica had already shown it was Mexico's equal, playing to a 0-0 tie during the group stage. In Sunday's semifinal, Philadelphia Union keeper Andre Blake made five saves to keep the Reggae Boyz even long enough for Lawrence to send them back into the Gold Cup final.
Two years ago, Jamaica beat the USA, 2-1, in the semifinals. Its run in the 2017 Gold Cup is all the more surprising in that it failed to reach the final round of World Cup 2018 qualifying.
After Jamaica's exit, German coach Winfried Schaefer was fired and replaced by former national team midfielder Theodore Whitmore, installed as head coach for the fourth time.
Jamaican soccer has been in transition since the death of longtime Jamaica Football Federation president, Captain Horace Burrell, who died after a long illness in June.
The conditions at the team's training facility at the University of West Indies, where it prepared for the Gold Cup, were so bad training sessions had to be cut short, and Blake, the captain, complained to Jamaica's sports minister.
Unlike the 2015 Jamaica team, which was built heavily around English-based players, including Leicester City captain Wes Morgan, Whitmore's team at the 2017 Gold Cup has only one European-based player: backup keeper Andre Miller from Swedish minor league club Syrianska FC.
The rest of the team consists of eight players based in MLS, four in the USL, plus 10 players from Jamaica's Red Stripe Premier League.
Jamaica was only the third Concacaf team to beat the USA in the Gold Cup when the Reggae Boyz won, 2-1, in the 2015 semifinals in Atlanta. The loss was the first of series of setback for the USA in the post-World Cup 2014 period that ended with U.S. coach Jurgen Klinsmann's firing in November 2016.
Just six starters from the 2015 match will be at Wednesday's final at Levi's Stadium:
Michael Bradley, Gyasi Zardes and Clint Dempsey for the USA and Je-Vaughn Watson, Darren Mattocks and Lawrence for Jamaica.
July 23 in Pasadena, California
Jamaica 1 Mexico 0. Goals: Lawrence 88.
Mexico -- J.Corona; Alvarez, Pereira, Molina, Ayala; Duenas, Gutierrez (Gallardo, 59), Pizarro; E.Hernandez Lopez, 79), Torres (Sepulveda, 46), Orbelin Pineda.
Jamaica -- Blake; Powell, Lowe, Taylor, Lawrence; Gordon, Watson, Richie, Francis (Morris, 53); Mattocks, Williams (Binns 77).