World No48 Walker, whose previous best performance at a Major was tied seventh at the 2014 US PGA, became the fifth consecutive first-time winner, finishing a shot clear of defending champion and world No1 Jason Day on 14 under. Third-placed Daniel Summerhays was a further three strokes behind.
The 37-year-old became just the seventh player and the first since Tiger Woods in 2000 to win the US PGA after leading, or being in a share of the lead, for the first three rounds.
Day, looking to become the first man to defend his US PGA title since Woods followed his 2006 win at Medinah with victory at Southern Hills a year later, saw his challenge effectively end with a missed putt for birdie at the 17th.
South Africa’s Branden Grace was left to rue missed opportunities along the back nine as he carded a three-under 67 in the closing round to end proceedings nine under. He finished tied for fourth alongside Hideki Matsuyama and Brooks Koepka.
Open Championship winner Henrik Stenson was vying to become only the second player after Ben Hogan to win back-to-back Majors after passing the age of 40 – Jack Nicklaus in 1980 Mark O’Meara in 1998 won two Majors in a year while in their forties but not consecutive events.
The Swede’s charge was impeded with a double bogey on the 15th, while the expected charge never materialised and he had to be content with finishing eight under. Germany’s Martin Kaymer matchd Stenson’s feat.
Tyrrell Hatton, who tied for fifth at Royal Troon last month, and world No30 Paul Casey were the highest-placed Britons. Both birdied the 18th to card final rounds of 68 and 67 respectively and finish seven under for the tournament.
Yorkshireman Danny Willett was crowned Masters champion in April but four successive rounds of scores in the seventies consigned the 28-year-old to US PGA obscurity as he finished tied for 79th on five over.
“It has been disappointing. A pretty stale week,” said Willett. “You try not to put expectations on yourself but you are only human. You know what you are capable of and if you don’t play at your peak, you get frustrated.”