American young gun Spieth sank a curling 28-footer to birdie the 16th and forge three ahead, then shrugged off a double-bogey at the 17th before becoming just the sixth player to slip into a green jacket and hoist the US Open trophy in the same year.
Though Spieth birdied the par-five 18th to end a wildly fluctuating final round of high drama with a one-under-par 69, he then had to watch as fellow American Johnson, in the last pairing, had a 12-foot eagle putt to win it at the 18th.
Johnson's putt slid past the cup and he then missed a four-foot birdie putt coming back to squander the chance of forcing an 18-hole playoff on Monday at Chambers Bay.
The 21-year-old Spieth posted a five-under total of 275 on a challenging links-style layout that firmed up under a baking sun as he became the youngest winner of the US Open since Bobby Jones in 1923.
“I'm still amazed that I won, let alone that we weren't playing tomorrow,” Spieth told reporters. “So for that turnaround right there, to watch that happen, I feel for Dustin.
“But I haven't been able to put anything in perspective yet. When I finished, I was just hoping to be playing tomorrow. I can't seem to wrap my head around the finish of today.”
The victory saw Spieth join Tiger Woods, Jack Nicklaus, Arnold Palmer, Ben Hogan and Craig Wood as the only players to have won the Masters and US Open in the same year.
“It's cool to be able to have two legs of the grand slam now, and to conquer golf's hardest test,” said Spieth. “The US Open is conquering the hardest layout in all of golf.
“The fact that we did it is amazing. We really grinded this week. Didn't have my best stuff ball-striking at all and we really grinded over those four or five-footers ... that was the difference.”
Spieth, who stunned the golf world with a wire-to-wire victory by four shots at the Masters in April, became the youngest player to win back-to-back majors since Gene Sarazen in 1922 (US Open and PGA Championship).
Johnson, seeking his first major crown, had to settle for a share of second place with South African Louis Oosthuizen, who sensationally birdied six of the last seven holes, including five in a row from the 12th, for a 67.
It was further major heartbreak for the big-hitting Johnson, a nine-times winner on the PGA Tour who came desperately close in his title runs at the US Open and PGA Championship in 2010.
South African Branden Grace, who's hopes evaporated with a double-bogey at the 16th where his tee shot ended up on railroad tracks out-of-bounds, carded a 71 to share fourth place with Australians Adam Scott (64) and Cameron Smith (68).
Johnson, one of four players tied for the lead overnight in the year's second major, appeared to have the title within his grasp when he moved two strokes clear at the turn on a picture-postcard afternoon in the Pacific Northwest.
However, he lost his way with bogeys at the 10th, 11th and 13th, leaving Spieth and Grace effectively duelling for the lead.
Spieth then benefited from a three-shot swing at the 16th after he sank a left-to-right breaking birdie putt from 28 feet and Grace ran up his double, before further drama unfolded over the last two holes.
World number one Rory McIlroy had earlier sent a tremor of excitement across the course as he racked up six birdies in his first 13 holes to reach two under for the tournament, just two off the lead.
The Northern Irishman drained a 70-footer at the par-four 13th to spark thunderous roars from the grandstands but bogeyed the 15th and 17th on the way to 66 and a tie for ninth at level-par.
“I hit some good shots early on, made some nice putts and just got a little bit of momentum going there,” McIlroy, who won the 2011 U.S. Open at Congressional by eight shots, told reporters.
“I feel like it's sort of one that got away, especially the way I putted this week. I don't think I've ever hit the ball as well in a major championship.”
Low scoring was plentiful, especially for the early starters, on a par-70 layout made more receptive by overnight watering and a set-up at its shortest yardage of the week.
The links-style venue became the longest course to stage a US Open when it measured 7,695 yards for the second round, but was cut back to 7,384 yards for the final round. – Reuters
Original source: Spieth seals back-to-back-Majors