Lauren Holiday and Tobin Heath had the other goals for the Americans, who claimed their third title after previously winning in 1991 and 1999.
"It's been amazing," said Lloyd. "We just wrote history today and brought this World Cup trophy home, which is unbelievable."
Julie Johnston put a ball in her own net in the second half, while Yuki Ogimi had the other goal for Japan, which won its only World Cup by beating the U.S. on penalties four years ago.
This one wasn't nearly as close.
In front of a raucous pro-American crowd of 53,341 at B.C. Place Stadium that included U.S. Vice-President Joe Biden, Lloyd opened the scoring in the third minute on a nicely worked corner kick, guiding home Megan Rapinoe's low drive after making a strong run to the penalty spot.
The American captain then made it 2-0 with her fifth of the tournament just two minutes later by poking home Holiday's free kick after it was flicked on by Johnston in a chaotic penalty area.
Holiday went from provider to scorer in the 14th minute, jumping on a terrible mistake by defender Azusa Iwashimizu to volley it home past helpless Japanese goalkeeper Ayumi Kaihori.
Lloyd then completed her hat trick two minutes after that on a goal that will be replayed over and over. She picked up the ball in her own territory and moved towards half before unleashing a shot towards goal. Kaihori stumbled as she tracked back and could only watch as the ball went off her hand, off the post and in.
"I've dreamed of scoring a shot like that," said Lloyd. "I did it once, I think, when I was a little bit younger on the national team in a training environment.
"Very rarely do you just wind up and hit it."
As the Americans on the pitch and in the stands celebrated, Kaihori lay on her back with her hands covering her face.
"We talked about trying to start fast," said U.S. head coach Jill Ellis. "That's been one of our mantras — start fast, finish strong."
When play resumed, Lloyd could have easily scored a fourth goal on a header moments later that went just wide.
Japan eventually steadied its ship and got one back in the 27th minute when Ogimi collected a ball in front of Hope Solo and beat the U.S. 'keeper with a high shot she could only get a finger to.
The goal was the first the Americans had conceded since their opening game, a span of 540 minutes.
Japan subbed on veteran midfielder Homare Sawa and striker Yuika Sugasawa later in the half, but the Americans took a 4-1 lead into the break.
Japan cut the lead to 4-2 in the 52nd minute when Johnston accidentally headed an Aya Miyama free kick into her own goal, but Heath got that one back just two minutes later on another scramble off a U.S. corner.
American striker Abby Wambach — a member of the national team for 15 years who has a record 183 goals for her country — has had a reduced role in her final World Cup, but came on in the 79th minute to a rousing ovation.
Downtown Vancouver was awash in red, white and blue in the days and hours leading up to the game as American fans descended on the city by the thousands. The crowd inside B.C. Place was no different, although there were pockets of Japanese supporters who did their best to be heard.
U.S. fans cheered and chanted "U-S-A! U-S-A!" 90 minutes before kickoff, and it only got louder as the afternoon wore on under a haze of smoke from forest fires raging across the province.
Note: The U.S. beat Japan for gold at the 2012 London Olympics. ... Sawa has said this will also be her last World Cup. ... England beat Germany 1-0 in Saturday's third-place match.
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