Mosco's goal with two minutes remaining in the extra period completed a third win in as many games for the reigning CONCACAF champion, and finished it just ahead of El Salvador, which earlier in the day beat Spain 3-2 on a last-minute goal by Tomas Hernandez.
Spain finished third with the United States last, having lost all three matches.
Angel Rodriguez and Yuri Morales traded goals in the final minute of the first period to leave Mexico and the United States tied 1-1 and send them to extra time.
In the first game Sunday, Hernandez scored twice in the final period - including his winner - to rally El Salvador from an earlier deficit to beat Spain. It was Hernandez's third goal of the tournament. Teammate Jose Agustin finished as the event's leading scorer with seven goals.
Agustin scored three and twice give El Salvador the lead on Friday over Mexico, which rallied to prevail on penalties 4-3 after a 4-4 draw. The game was a rematch of the 2010 CONCACAF championship final, when Mexico beat Los Cuscatlecos 5-3.
Spain 4, United States 2
Mexico 4, El Salvador 4 (Mexico wins 4-3 on penalties)
El Salvador 8, United States 6
Mexico 4, Spain 1
Spain 2, El Salvador 3
Mexico 2, United States 1