The Central Americans eliminated the reigning world and European champions with a 1-0 win at St James' Park, putting themselves in a strong position to qualify from the group stage of the competition.
Mendoza made a number of match-saving blocks to lead his team to victory, but feels that the supporters' contribution was invaluable.
"We could hear the fans supporting us," Mendoza told reporters.
"It was really a surprise and it definitely gave us strength. I'm delighted that we got this win, this is also for [the fans].
"We know how important it is to beat Spain and get these three points, but the important thing will be not to lose concentration as the next game comes soon."