Horan scored nine goals to lead the United States at the 2010 CONCACAF U-17 Championship and is one of six members of that side to graduate to this year's U-20 team. The Americans outscored opponents 38-0 at the U-17s two years ago but failed to qualify for the World Cup.
Horan gave the United States the lead in the 12th minute, and added goals in the 47th and 74th to lead a dominating performance, the American's 21st victory in 24 games at the CONCACAF finals.
With temperatures hovering around 90 degrees (32 Celsius) and 43 percent humidity at kickoff, the match appeared to start slowly. But the United States earned two corners in the first eight minutes and had a goal by Morgan Brian disallowed in the eighth because of offside.
The Americans finally scored when Crystal Dunn curled a cross to the back post, where Horan outleapt Gabriela Mejia and Marily Rivera just outside the six and headed it down. The ball bounced once before Guatemala keeper Stephanie Castellon was able to get both hands to, but it deflected over her before she could reach back and scoop it out of the goal.
It took 18 minutes before Idiana Perez was able to get Guatemala's first shot, a weak, rolling effort from distance, and the United States doubled its lead 11 minutes later.
Kealia Ohai's cross was headed by Guatemala defender Waleska Castaneda to the top of the area, where Johnston ran onto it and struck it first time to put the Americans ahead 2-0.
The Americans earned six corners in the first half and hit the right post on a shot by Brian in the 41st, but needed until the second half to add to its lead.
Horan ran onto a through ball from Brian down the left side, took a touch into the area and beat Castellon with a flick of her right foot.
Kathie Stengel increased the lead to 4-0 in the 54th, heading in a cross from Dunn at the corner of the goal box, and the United States virtually sealed the victory in the 60th when Cari Roccaro shot from just outside the 18 and the ball appeared to slightly change direction in the middle of the area past Castellon.
Horan completed the scoring with a little acrobatics. Chi Ubogagu sent in a cross that Maya Hayes headed off the cross bar from inside the six. The ball bounced off Mejia onto the post and back in front of the goal, where Horan stuck it with a side scissors kick from the edge of the goal box to make it 6-0.
Guatemala, making only its second appearance at the CONCACAF U-20 women's finals, will try to recover on Sunday when it faces host Panama. The Americans will play Cuba earlier in the day.