The win, in the shadow of the doping suspensions which they claimed was the result of contaminated food, ensured Mexico advanced to the quarter-finals.
Andres Guardado scored twice during a first-half assault that gave Mexico a 4-0 advantage by halftime. Rafael Marquez and Pablo Barrera also scored before the break.
Marcos Urena scored in the 68th minute for Costa Rica, who will also advance to the quarter-finals after finishing tied for second in Group A on four points with El Salvador but have a better goal difference.
Costa Rica were tied on four points - the same as El Salvador, who had earlier thumped Cuba 6-1, - but qualified due to a better goal difference.
El Salvador still could advance to the quarter-finals as one of the two-best third-placed sides.
Andres Guardado scored twice during a first-half assault that gave Mexico a 4-0 advantage by halftime. Rafael Marquez and Pablo Barrera also scored before the break.
Marcos Urena scored in the 68th minute for Costa Rica.
El Salvador's Rodolfo Zelaya had earlier scored a goal in each half to lead his country to a 6-1 rout of Cuba and their first victory of the tournament.
El Salvador took a 2-0 lead at halftime courtesy of goals from Zelaya and William Romero before Mark Blanco, Arturo Alvarez and Eliseo Quintanilla all scored in the second half.
Cuba, who had been thrashed 5-0 in both of their previous games, scored a consolation goal through Yenier Marquez in the 82nd minute.
