Koepka, who became the seventh straight first-time victor of a major, finished the tournament strong, closing with three straight birdies on the back nine and shooting a 5-under 67.
It was hard to find much fault with what Harman did Sunday. He earned $2.16 million, his nation's championship and the comfort of knowing the far-flung trips to Kenya and Kazakhstan and India and beyond are all in the rearview mirror.
Koepka parred the final two holes to tie the U.S. Open's 72-hole scoring record (in relation to par).
Koepka said the world number one had called him on Saturday, offering some advice which came in handy.
HARMAN: No, I don't think there's - I think most of the good players don't really compare themselves to their peers. "It hasn't sunk in, obviously, and (it) probably won't for a few days".
Fleetwood, who was hoping to become the third English victor since 1924 after Tony Jacklin (1970) and Justin Rose (2013), began the day in a tie for second with Thomas and Brooks Koepka, a shot behind surprise leader Brian Harman.
England's Tommy Fleetwood finished in fourth place one shot back.
"To finish in double digits under par at a major championship, especially the Open, it was a good week", Fowler said.
He closed with a 5-under 67, only realizing after his par on the final hole that a birdie would have set yet another U.S. Open record in a week filled with them. He's in the British Open field and felt he'd also qualify for this season's PGA Championship.
Koepka had always been thought of as a potential major champion and showed exactly why on Sunday with a near flawless performance, which saw him defy gusting winds early in his round and then capitalise on the return of calmer conditions on the back nine.
But Koepka made a par-saving putt on No. 13 and then birdied the next three holes to take all the suspense out of what had been shaping up to be a dramatic finish. He bogeyed the hole. "I've done it before, but he did it today".
"The way I putted this week was unbelievable too". "I felt like it never fully came together", Koepka said. I was just trying to get in as low as I could. As Harman began to fade, Koepka poured it on with birdies over the next three holes, lightly pumping his fist after each one.
"I don't believe in moral victories", Harman said. It was just the third time he made the cut.
Brooks Koepka was exhausted of being called an underachiever.
"It's unveliveable. I dont' even know what to say, it's pretty cool", Brooks said after his victory.
"It is Father's Day, so hopefully - like I said, I didn't get him a card, so I really hope this works", Koepka said.
Going into Sunday five players had carded rounds of seven-under or less, the most ever in the U.S. Open's 117-year history while Canadian Adam Hadwin fired off six consecutive birdies in Thursday's opening round to equal another mark.
Si Woo Kim, the 21-year-old Players Championship victor who began a compelling three shots off the lead, shot 75 and finished at a commendable 6 under par.
Another major, another maiden champion.
Brooks Koepka kisses the winning trophy after the U.S. Open golf tournament Sunday at Erin Hills in Erin, Wis. "I don't think I even mentioned to my caddie, Ricky, even about winning all week", Koepka said.
So, I think I didn't really understand the beef that it was getting for the low scores.