They will face the winner of the other semi-final later on Wednesday between Mexico and Panama.
The 2013 champion Americans had reached five successive Gold Cup finals and were coming off a 6-0 thrashing of Cuba in the quarter-finals.
Mattocks, who was suspended for his team's quarter-final win against Haiti last week, struck in the 31st minute with a header before Barnes scored from a free-kick five minutes later.
The U.S. had made defensive changes entering the match, as John Brooks returned from suspension and replaced Omar Gonzalez while Brad Evans replaced Timmy Chandler, but they did not pan out.
American captain Michael Bradley helped claw the home side back with a goal in the 48th minute.
The U.S., however, could not find the equaliser despite pressuring Jamaica goalkeeper Ryan Thompson for much of the second half.
(Writing by Jahmal Corner in Los Angeles; Editing by Greg Stutchbury)
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