El Tri needed only a single Jonathan Dos Santos strike to secure continental glory when the teams met in the Gold Cup final in July, but they went two better on Friday.
Sevilla striker Hernandez headed home during the first half and substitutes Erick Gutierrez and Uriel Antuna completed a thoroughly deserved victory with two goals in four second-half minutes.
USA's brightest spark, Christian Pulisic, passed up the chance to convert a late penalty, which young forward Joshua Sargent was unable to convert past Jonathan Orozco.
Sergino Dest made his USA debut at left-back and forced goalkeeper Orozco into the first real save of the match with a threatening long-range effort.
The Ajax teenager was less convincing when Jesus Manuel Corona nutmegged him, advanced to the byline and picked out Hernandez to head in the 21st-minute opener.
Mexico should have extended the margin before the break, but Porto winger Corona failed to punish a defensive turnover.
Gregg Berhalter turned to in-form Werder Bremen forward Sargent after the interval, but the visitors continued to look the more dangerous.
Corona dragged a shot wide of the far post following a quick counter-attack and substitute Hirving Lozano did likewise in the 71st minute.
The second eventually arrived via a USA mistake as Zack Steffen's ill-conceived pass from the last line of defence was picked off by Gutierrez, who made no mistake with a simple finish.
Orozco's post prevented Alfredo Morales from reducing the deficit moments later and Antuna rubbed salt into the wound eight minutes from the end, coolly beating Steffen after being expertly freed by the lively Lozano.
There was time left for Sargent to be sent to the spot when Diego Reyes bundled over Josh Morris, however Orozco guessed correctly and pushed the 19-year-old's penalty away from the bottom left corner.
What does it mean? USA very much a work in progress
USA boss Berhalter clearly wants his team to build from the back as often as possible, a style which regularly invited trouble against a canny Mexico. The upcoming CONCACAF Nations League serves as a good opportunity to smooth out the flaws of a gameplan which needs refinement ahead of the main event that is World Cup qualifying.
Lozano oozes class in cameo
He spent just 20 minutes on the pitch but that is all Napoli's new signing needed to make a major impact. Lozano bamboozled a couple of markers on the way to setting up his side's final goal and generally looked to be the game's most talented individual, even above the hard-working but luckless Pulisic.
Dest's difficult debut
USA elevated defender Dest into their senior squad in an attempt to ward off interest from Netherlands, the country of his birth and whom he could still represent in future. On the evidence of some naive moments defensively in this match, the 18-year-old has a lot to learn at international level regardless of the shirt he is wearing.
Both teams return to action on Tuesday. USA face Uruguay in St Louis, while Mexico head to San Antonio to meet Argentina.