The Whites travelled to the East London for the second weekend in a row after suffering a controversial FA cup exit in an identical head-to-head the previous week.
Though the Hammers' 2-0 win inflicted a fourth consecutive third-round exit on Leeds, Marcelo Bielsa's side had a sharper focus on the Premier League encounter after a challenging first half of the season left Leeds lingering near the drop zone and in need of points.
Three thousand Leeds fans erupted with delight as Harrison opened the scoring for the Whites in the tenth minute of the top-flight clash.
United's early form looked to be unsettled, though, as Adam Forshaw and Junior Firpo were both forced off the pitch by injury around the twenty-minute mark, to be replaced by 18-year-old substitutes Leo Hjelde and Lewis Bate, each making their Premier Leagued debut.
With his fourth goal in as many games, Jarrod Bowen made it 1-1 with a header for West Ham before Harrison retook the lead for Leeds ahead of half time.
Pablo Fornals equalised for the home side in the 52nd minute but Harrison was there again to prevent the scores staying level long, running onto an inch-perfect through ball from Raphinha to deliver a composed finish past Łukasz Fabiański.
Both sides saw goals chalked off in the final quarter of the game, with Mateusz Klich and Irons substitute Andriy Yarmolenko denied by a VAR offside ruling.
In injury-time, the in-form Bowen missed a gilt-edged opportunity to steal a point for West Ham, sending the ball careering over the bar.
Leeds left London with all three points, and Harrison claimed the match-ball on a stellar afternoon away from home.
Here are the photos that tell the story of the game:

13. West Ham United v Leeds United
Jack Harrison beats Łukasz Fabiański to complete his first career hat-trick. Photo: Alex Pantling

14. West Ham United v Leeds United
Lewis Bate and Dan James celebrate with the Whites' hat-trick hero after Jack Harrison gives Leeds the lead again in the 59th minute. Photo: Alex Pantling

15. West Ham United v Leeds United
Mateusz Klich celebrates putting the Whites 4-2 ahead. Photo: Mike Hewitt

16. West Ham United v Leeds United
Mateusz Klich's goal is chalked off by VAR. Photo: Alex Pantling

17. West Ham United v Leeds United
Substitute Andriy Yarmolenko appears to have equalised for the Irons before VAR finds an offside. Photo: Alex Pantling

18. West Ham United v Leeds United
Dan James challenges Łukasz Fabiański as Leeds push to hold on for the win in the last ten minutes. Photo: Mike Hewitt

