Diego Luna, who is of Mexican descent, scored his first international goal in the quarter-finals against Costa Rica
The United States will face holders Mexico in the Gold Cup final after both won their semi-finals on Wednesday.
The US beat Guatemala 2-1 in St Louis, with Real Salt Lake forward Diego Luna scoring twice in the opening 15 minutes.
Mexico booked their spot in the final when Fulham striker Raul Jimenez scored the only goal in a 1-0 win over Honduras in Santa Clara, California.
The US, who are co-hosting the tournament with Canada, got off to a fine start when Luna pounced to put them ahead on four minutes after Guatemala goalkeeper Kenderson Navarro parried a shot from Luca de la Torre.
US midfielder Malik Tillman then went close before Luna collected the ball about 30 yards from goal, ran at a defender and cut inside him before firing a right-footed strike into the bottom corner from the edge of the box.
Rubio Rubin and Jose Morales were denied by home goalkeeper Matt Freese before 18-year-old forward Olger Escobar gave Guatemala hope when he fired into the bottom corner with 10 minutes remaining.
But the US held on to give coach Mauricio Pochettino the chance to claim just the third major honour of his managerial career.
The seven-time winners had come into the tournament on a four-match losing streak, with Pochettino leaving out star player Christian Pulisic after the AC Milan forward offered to play in two friendlies before the Gold Cup, but wanted to be rested for the tournament itself.
Mexico reach ninth final out of 12
Mexico are chasing a record-extending 10th Gold Cup title at Houston's NRG Stadium in Texas on Sunday.
They were in control for much of their last-four game against Honduras.
Defender Johan Vasquez had a close-range effort blocked, while Honduras goalkeeper Edrick Menjivar made a fine save to deny Marcel Ruiz shortly before half-time at the Levi's Stadium.
But the defending champions made the breakthrough five minutes into the second half when 16-year-old midfielder Gilberto Mora jinked inside and laid off for Jimenez to fire low past Menjivar for his 42nd international goal.
It was the 34-year-old's second goal of the tournament, having scored in the opener against Dominican Republic, which took place amid tensions over US president Donald Trump's immigration crackdown.
West Ham midfielder Edson Alvarez, the Mexico captain, had a goal ruled out for offside but one proved enough for his nation to reach a 12th Gold Cup final - and ninth from the past 12 editions.
