The 26-year-old from North Carolina, in just his second US Open, battled back from two early bogeys to grab four birdies in five holes from the sixth and then pared his way down the rest of the back nine for a gripping win.
Simpson closed with a 68 and a one-over total of 281, one stroke clear of countryman Michael Thompson (67) and Northern Ireland’s Graeme McDowell (73).
A further stroke back in a five-way tie for fourth place were Americans Jim Furyk (74) David Toms (68), Jason Dufner (70) and John Peterson (70) along with Ireland’s Padraig Harrington (68).
Simpson was the ninth straight first time winner in the majors and he was the 15th different winner in the last 15, underlining once again the current strength-in-depth of world golf.
“If I was honest with you I believed in myself I could win a major, but maybe not so soon,” he said.
“This is my fourth or fifth [major tournament]. And I just gained all the respect for the guys who have won multiple majors, because it’s so hard to do. The level of pressure is so much greater than a regular event.
“For me to play [on] Sunday fourth from the last group was probably a huge help as opposed to the last group.”
It was day when birdies were rare and survival was the order of the day as a cold mist and spotty rain drifted in from the adjacent Pacific Ocean to compound the difficulties of the exacting Lake Course layout.
Former winners Furyk and McDowell started the day holding the joint lead – the only men to be under par at one-under 209.
First blood went to Furyk as he took the solo lead after McDowell bogeyed the third and when the Ulsterman had a second bogey at the fifth, the Ryder Cup stalwart had a two-stroke lead.
He missed a 10 footer at the eighth that would have nudged him three strokes ahead and then, in his rear-view mirror, he saw a trio of young American countrymen – Simpson, Thompson and Peterson – making a charge at him.
First round leader Thompson set the early clubhouse standard with a 67 for a two-over total of 282 and sat back to watch if anyone could match him.
Surprisingly, Furyk started to unwind.
He bogeyed the 13th and then flung his club away in disgust after hitting a snap hook off the tee on the par-five 16th.
He salvaged a bogey from out of the trees but up ahead Simpson negotiated a tricky chip from deep greenside rough to save par at the last and that, coupled with the bogey for Furyk, left him one clear.
Furyk needed one birdie on the last two holes to force a playoff with McDowell needing to make up two.
The Irishman birdied the 17th, sending the final pairing to the final hole with both in search of the birdie they needed.
Furyk’s chances evaporated when he plugged his approach shot in a bunker, But McDowell hit to 24 feet above the hole only to see his snaking, downhill putt to get into an 18-hole playoff slip agonisingly past.
Waiting nervously in the clubhouse, Simpson hugged his wife Dowd when McDowell’s putt slid past and punched his fist in the air as the realisation hit him that he had won the US Open.
Furyk said he had felt all calm all day, but had been unable to produce the golf he was capable of.
“I don’t know how to put that one into words, but I had my opportunities and my chances and it was right there,” he said.
“On that back nine, it was my tournament to win and I felt like if I shot even par or one under I would have distanced myself from the field and I wasn’t able to do so.
“And I played quite well, actually until the last three holes.”
McDowell said that the bad, nervy start had left him with too much to do later in the round.
“Yesterday I managed to shake off my fears and nerves and go out and really kind of get in great groove like that,” he said.
“Today was a struggle. Conditions were tougher, the ball wasn’t flying very far.
“This golf course doesn’t allow you to get in a rhythm and today was a grind, it was a slog, and I was just happy the way I hung in and made a few birdies.
Earlier in the day, Tiger Woods saw his hopes of a 15th major title, four years after his last US Open win, rapidly crushed as he dropped six strokes in the first six holes.
The 36-year-old American eventually came in with a 73 and that, alongside his 75 on Saturday, was his worst weekend score relative to par in a major, casting fresh doubts over the current state of his game. – Sapa-AFP