After a scrappy opening to the game, USA notched the first try after seven minutes when Joseph Paulo offloaded in the left-hand corner to Kristian Freed, who managed to get the ball down.
That was followed up by Taylor Welch, who extended the lead to 8-0 as Scotland pushed for a way back into the match.
The Bravehearts suddenly drew level, however, with two tries in as many minutes. First Brett Phillips stormed through a hole in the USA defence to score, and then Matthew Russell's run resulted in another. Danny Brough then booted over the conversion.
Prop Luke Douglas then found a huge hole in the USA defence to score Scotland's third try in 12 minutes.
Alex Hurst added gloss to scoreline to complete the victory.