Miles Robinson headed in the 118th-minute winner as the United States won another title after June's CONCACAF Nations League success.
Mexico missed a second-half penalty before Hector Herrera netted late to keep their bid for a 12th Gold Cup alive.
Canada were rocked by the earliest goal that the United States have ever scored in a Gold Cup game.
The contest is being built up as one which could be the most evenly contested matches between America and Canada in recent times.
Costa Rica will be favourites to secure their second consecutive win of the tournament.
Rogelio Funes Mori stepped up for Mexico in the absence of injured star Hirving Lozano as they eased past Guatemala on Wednesday.
The 16-team tournament was pushed back from Jul 2 to July 10 and will be played until August 1, 2021.
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