Mexico's all-time leading scorer was well marshalled by South Korea's defence for much of Saturday's clash in Rostov-on-Don, but took his chance when it came in the 66th minute, slotting home in composed fashion at the culmination of a wonderful counter-attack.
Hernandez's goal was nothing less than Mexico, who beat defending champions Germany 1-0 last time out, deserved following another accomplished performance, with South Korea having rallied following Carlos Vela's 26th-minute penalty.
It was not always smooth sailing for Juan Carlos Osorio's side, though, with Lee Yong and Son Heung-min passing up presentable opportunities in the early stages.
However, Jang Hyun-soo's handball gave referee Milorad Mazic little choice but to point to the spot – the number of penalties awarded in Russia consequently surpassing the 13 across the entirety of the previous World Cup – and Vela, who announced the death of his grandfather following the win over Germany, converted.
Hernandez's strike means Mexico will progress to the last 16 if Germany fail to beat Sweden, with Son Heung-min's added-time stunner not enough to stop South Korea staring at a potential group-stage exit.
Hirving Lozano was Mexico's hero on matchday one and the winger proved his worth at the other end in the 12th minute with a superb block to deny Yong.
South Korea threatened again nine minutes later, Son breaking clear of Mexico's line before letting rip with a low drive that Hector Moreno did well to keep out.
Mexico showed their quality on the break again when Miguel Layun fired in a cross that Lozano very nearly got on the end of, but they did not have to wait much longer to take the lead.
With South Korea's defence in disarray, Jang clearly handled Andres Guardado's cross and referee Mazic did not hesitate in awarding the penalty, Vela stepping up and sending Cho Hyun-woo the wrong way.
14 - Carlos Vela's penalty was the 14th taken at the 2018 World Cup; already one more than in the entirety of the 2014 tournament (13). Spotted. #MEXKOR #MEX #WorldCup pic.twitter.com/8GcWYpil9O — OptaJoe (@OptaJoe) June 23, 2018
Vela almost turned provider two minutes later with a neat ball into Layun, but the former Watford man lashed his effort over from close range.
South Korea wanted a penalty of their own after the restart when Moon Seon-min's strike clipped Carlos Salcedo's arm, but the referee correctly waved away their appeals.
Ki Sung-yueng was the next to test Mexico's resolve, stinging the palms of Guillermo Ochoa with a rasping first-time strike.
Not to be outdone, Cho was at his best soon after, South Korea's goalkeeper lunging to his right to tip Hernandez's goal-bound shot wide.
Hernandez would not be denied again, though, the West Ham striker making no mistake in drilling in his landmark goal to send the raucous Mexican crowd into raptures.
Son's sublime 20-yard strike into the top-left corner punctured Mexico's defence in the third minute of stoppage time, but it would prove little more than a consolation as South Korea slumped to a second successive defeat.
Key Opta stats:
- South Korea have lost their last four World Cup matches; their joint-worst losing run at the World Cup, also losing four between 1986 and 1990.
- In the first half, Son Heung-min had six shots (including blocked attempts) – more than South Korea managed in the entirety of their first match against Sweden.
- Javier Hernandez is only the third player to score at three different World Cups for Mexico (2010, 2014 and 2018) after Cuauhtemoc Blanco (1998, 2002 and 2010) and Rafael Marquez (2006, 2010 and 2014).
- South Korea have conceded a goal via a penalty in both of their World Cup games so far in 2018 – in their previous 29 World Cup games combined, they only conceded one penalty goal.