By Mark Lamport-Stokes
Three strokes behind American playing partner Dustin Johnson going into the final round, the Northern Irishman ground out a three-over-par 74 at a breezy Pebble Beach Golf Links.
Despite dropping three shots on the more difficult back nine as the ocean winds strengthened in the late afternoon, McDowell held his nerve with some of the game's biggest names close behind to finish at even-par 284.
"It's so difficult to win a golf tournament, let alone a major," a beaming McDowell said greenside after kissing the silver trophy and then cradling it like a baby. "This thing feels amazing.
"I've dreamed of this all my life, two putts to win the U.S. Open. A few pints of Guinness will probably go down around now," he said of his fans back in Northern Ireland watching on television.
Frenchman Gregory Havret, who came through 36 holes of sectional qualifying last month to book his place at Pebble Beach, closed with a 72 to secure a surprise second place at one over in his first U.S. Open.
Double U.S. Open champion Ernie Els of South Africa, briefly tied for the lead early on, was a further stroke back after a 73 with world number one Tiger Woods (75) and fellow American Phil Mickelson (73) joint fourth at three over.
"I made three mental mistakes and it cost us a chance to win the U.S. Open," said the American who won the championship by a record 15 shots when it was last held here in 2000.
McDowell was hugged by his father Kenny after sealing victory on the 18th green, having posted the highest closing score by a U.S. Open champion since Andy North in 1985.
"I had a little peak at the leaderboard to see what was going on and no one was going crazy. I just tried to stay calm on the back nine and I really did. I did a great job of it."
Winner of the European Tour's Wales Open two weeks ago, McDowell took advantage of a nightmare start by Johnson to take control of the final round.
Johnson, who had won the previous two PGA Tour events staged at Pebble Beach, began the day with a commanding three-stroke lead which he spectacularly threw away by dropping seven shots in six holes.
He triple-bogeyed the par-five second after taking three shots to escape thick fescue grass above a greenside bunker and double-bogeyed the third after a hooked tee shot led to a lost ball.
Two behind McDowell at that point, Johnson went on to bogey the par-four fourth after pushing his tee shot into the ocean as he slumped to an 82 for a share of eighth place at five-over 289.
McDowell, who made a series of clutch putts to save par on the front nine, picked up his only birdie of the day when he knocked in a 10-footer at the par-three fifth.
However he bogeyed the tricky ninth, the second hole in the stretch dubbed the "Cliffs of Doom," after finding a greenside bunker with his approach to reach the turn in level-par 35.
Although he also bogeyed the 10th, 14th and 17th, McDowell was given welcome breathing space when his closest challenger, Havret, also bogeyed the 17th to remain a stroke behind.
Havret narrowly missed a birdie putt from nine feet on the 18th green, and McDowell safely parred the hole after laying up with his third shot and two-putting.
"It has been a great week," said the Frenchman, whose world ranking rocketed from 395th to 104th with his runner-up finish. "Of course I'm a bit disappointed now because I've been so close to heaven."
(Editing by Alastair Himmer)