The last man to achieve such a feat prior to Harrington was Scotland’s Tommy Armour in 1930, when the competition was still played under the match play format before its switch to stroke play in 1958.
The triumph was part of a golden year for Harrington.
The Irishman had already won the Open Championship before becoming the first European to win back-to-back major championships with his victory at the US PGA.
He was also named PGA Player of the Year, PGA Tour Player of the Year and European Tour Golfer of the Year in 2008.
The three-time major champion’s success sparked an era of European renaissance in the year’s final major with Germany’s Martin Kaymer claiming the top prize in 2010 before Harrington’s fellow Irishman Rory McIlroy triumphed in 2012 and 2014.
Harrington hit a second-round 74 at Oakland Hills in 2008 and admitted that he felt beaten but he fought back for the most unlikely of wins.
The then 36-year-old was locked in a tight battle with Spaniard Sergio Garcia but he saw off the challenge of his continental compatriot to claim his third major title, just a month after winning his second Open at Birkdale.
American Ben Curtis had led the field on Sunday morning, one shot ahead of fellow countryman JB Holmes and Sweden’s Henrik Stenson.
Garcia and Harrington started the day one over par but a triple bogey from Holmes and three bogeys in four holes from Curtis ended their chances of victory.
Harrington birdied the 17th to take the outright lead for the first time as Garcia’s putt lipped out on the same hole. The Irishman managed to par the last after a shaky tee shot saw him find the bunker. But he recovered with a calmly-taken 15-foot putt. Garcia’s first shot at the 18th proved just as poor as he hit the rough before finishing with a bogey.
Curtis needed an eagle on the final hole but that proved a stretch too far as Harrington ended Europe’s 78-year wait.
“I really do like the fact that no other European has won two majors consecutively, because I hold a lot of European players who I grew up watching in high esteem,” said Harrington, in reaction to his 2008 win.
“To achieve something that they hadn’t is very special.”
Harrington is no longer the only European to hold such an honour after McIlroy achieved the same feat in 2014, winning the Open prior to his second PGA success.
The Northern Irishman is among the favourites for this year’s tournament, along with Justin Rose, who was tied ninth back in 2008. England have never had a US PGA winner since the competition’s switch to the stroke play format.
The 100th PGA Championship tees off on Thursday.