McIlroy topped off an incredible summer by securing his second USA PGA Championship, only three weeks after winning his first British Open.
By the final hole, there were still three players in with a shout of winning the tournament and they were forced to play in near darkness due to a two-hour rain delay earlier in the afternoon.
McIlroy had gone into the final day one shot clear of his nearest opponent, but on the front nine trailing to the leaders by three shots.
Americans Phil Mickelson and Rickie Fowler were paired in the group in front of McIlroy, and had a huge home following which spurred them on to get eight birdies between them over the first nine holes.
McIlroy fought back with the form he is known for of late with a tenth hole eagle and birdies at the seventh, 13th and 17th so a par putt at the 18th was all that was needed to secure his fourth major.
The win means that he is the first golfer to win back-to-back majors since Padraig Harrington had a brilliant year in 2008, following his Masters win with later consecutive British Open and PGA wins that summer.
The Holywood golfer only managed to complete his round on Sunday due to Mickelson’s sportsmanship in allowing the World No.1 to catch up with play as darkness fell.
The runner-up place for Mickelson means he qualifies for the US Ryder Cup team whereas Rickie Fowler broke a record of his own – becoming only the third man to finish in the top five of all four majors in a season in the modern era, joining Jack Nicklaus (1971 and 1973) and Tiger Woods (2000, 2005).