Walker had held at least a share of the lead at the end of each day during the final Major of the year in New Jersey but he had to produce a flawless last 18 holes to see off the challenge of world number one and defending champion Jason Day.
After a lengthy rain delay on Saturday, the leading players were forced to play 36 holes on Sunday and Walker had established a one-shot lead by the end of his third round.
He did not start his final 18 holes until 3.16pm local time but his consistent play ultimately proved key as Day started his round with a couple of bogeys.
Walker's first birdie at the 10th hole came in spectacular style as he holed a shot from the bunker before he found the cup from 25 feet at the next to move clear of Day.
Ahead of the final two holes - the only two par fives on the course - Walker held a two-shot lead and that became three shots when he birdied the 17th.
However, moments later, Day registered an eagle at the last to leave Walker requiring a par but despite finding the rough with his second shot, he drained a three-foot putt to claim his first Major crown.