The former Swansea player came off the bench for the second half and scored in the 89th minute, sparking delight in the visitors' ranks.
Spanish midfielder Hernandez found the bottom left corner after Luke Ayling cut the ball back, just as Leeds looked like being frustrated.
Now they sit three points clear of second-placed West Brom and six ahead of in-form Brentford, who occupy third spot, and only three rounds of games remain.
Leeds have been absent from the top flight of English football since their relegation in 2004, which came just three years after a run to the Champions League semi-finals.
Veteran manager Marcelo Bielsa was persuaded to stay for this season, having gone agonisingly close last term to the elusive promotion.
Now, though, Leeds look ready to end their long wait, with games against Barnsley, Derby County and Charlton Athletic to come and a maximum of four points required.
The latest step on the road back to the elite came a day after the death was announced of one of their greatest players, England World Cup winner Jack Charlton, who spent his entire club career with Leeds.
Stoke City and Birmingham City have played Premier League football more recently than Leeds, but this season has been one of battling the threat of dropping into the third tier for both.
Stoke earned a 2-0 win over Birmingham on Sunday to join the Blues on 49 points, with both sides now four points clear of the relegation zone.
Beaten 5-0 by Leeds in their previous outing on Thursday, Stoke bounced back thanks to goals from Danny Batth and captain Sam Clucas.
Yet the top two in England's second tier look increasingly likely to be the pair promoted to the Premier League, with third-placed Fulham suffering a damaging home defeat against Brentford.
In the first matches following the league's three-month coronavirus suspension, a new hero emerged for mid-table Derby County as 18-year-old Louie Sibley scored a hat-trick in their win at Millwall.
And Rhian Brewster, a striker on loan from Liverpool, gave Swansea City's play-off hopes a boost by netting twice at Middlesbrough, as well as making a powerful political statement.
POINT A MIXED BLESSING FOR BAGGIES
With Leeds in action at Cardiff City on Sunday, West Brom had an opportunity to go two points clear of Marcelo Bielsa's team as the league resumed.
Bilic's players dominated the derby clash and had a host of chances but could not find a breakthrough, and a stalemate at the Hawthorns potentially plays into the hands of Leeds.
The top two are now level on points, with West Brom top on goal difference, but Leeds will have the chance to go three clear before the next round of games.
BEES STING NEIGHBOURS
Said Benrahma and Emiliano Marcondes scored late for Brentford in a 2-0 win at promotion rivals Fulham, a result that leaves West Brom and Leeds seven points clear with eight games remaining for the play-off pack.
Nottingham Forest were heading for victory at Sheffield Wednesday after Joe Lolley gave the visitors a 69th-minute lead, only for Connor Wickham to grab a stoppage-time leveller for the Owls.
Forest sit fifth, four points clear of a Preston North End side who were held 1-1 at Luton Town, Callum McManaman firing a late equaliser for the hosts, who nevertheless fell to the foot of the table.
KING LOUIE
Blackburn Rovers beat Bristol City 3-1 in a battle between teams fighting with Preston for sixth place, but the individual performance of the day came from Sibley, whose heroics guided Derby to their 3-2 win at Millwall.
"I'm absolutely buzzing," Sibley told Rams TV. "To get that hat-trick was just unbelievable. When the third goal went in, I couldn't stop smiling."
Brewster was not far behind, his double steering Swansea to a 3-0 victory at lowly Middlesbrough.
Brewster held up a shirt that read "our colour is not a crime" after his first goal, expressing support for the Black Lives Matter movement.
TIGERS ON THE SLIDE
Wigan Athletic were 2-0 winners at fellow strugglers Huddersfield Town, while Hull City slipped into the bottom three after losing 1-0 at home against Charlton Athletic.
Barnsley climbed off the bottom by winning 1-0 at QPR, and Stoke City drew 1-1 at Reading.
The Terriers avoided a defeat that would have seen them relegated on Saturday, but even a win on the final day over promotion hopefuls Ipswich Town will not be enough unless they overcome a huge goal difference of 15 goals with Plymouth Argyle.
Birmingham remain in the other relegation place and now must beat Norwich City to stand a chance of staying up with Sheffield Wednesday, Plymouth and Blackburn Rovers also still in danger.
At the other end of the table, the Canaries dropped points in a 2-2 draw with Swansea City, leaving the door open for Hull City, who face Ipswich Town later today.
