It was all too much for the rest of the field, including Australia's Gabriela Ruffels, who birdied five of her last six holes on Saturday to get within two shots of the lead. She made only one birdie in her round of 76, finishing 11 shots adrift of the leader.
Iwai led from the opening hole at El Camaleon when she made birdie and Jenny Bae made bogey, and the 22-year-old from Japan was on her way to her first LPGA title.
"My mental is so calm, calm every day, every time," Iwai said.
She ran off four straight birdies, poured in every putt that mattered and then cruised home to become the third LPGA rookie to win this season.
Iwai, whose twin sister Akie Iwai has been runner-up twice this year, finished with a 12-under 276.
Bae had a one-shot lead going into the final round at Mayakoba, and the 10 players behind her also had never experienced winning on the LPGA Tour.
But it did not take long for Iwai, who had eight wins on the Japan LPGA before getting her LPGA card, to turn it into a rout.
She hit wedge to four feet on the third hole for birdie. She rolled in a seven-foot birdie putt on the fourth. Her approach to the par-5 fifth was right of the green, close to the water, and she calmly chipped that to two feet for a third straight birdie. Iwai finished the big birdie run with a 15-foot putt on No.6.
Bae bogeyed the opening two holes and never caught up. She steadied herself on the back nine to salvage a 73 and a runner-up finish.
"It just tells me that I have a long ways to go but also I'm also getting there, too," Bae said. "So I'm happy. I definitely know what I need to work on for the week off for me. Hopefully I'll be able to score better at the next tournament."
The six-shot victory for Iwai matched the largest margin this year. Rio Takeda, of Japan, also won by six at the Blue Bay LPGA in China.
Iwai took off early and it seemed as though everyone simply cleared out of her way.Â
Miranda Wang, who stayed in the game with clutch par putts down the stretch in the third round, closed with a 76 without making a birdie. Yahui Zhang made bogey on six of her last seven holes for a 78.
Robyn Choi was the highest-placed Australian, a final-round 70 giving her a share of ninth place, a shot ahead of Karis Davidson and two in front of Ruffels. Sarah Kemp finished six over after a final-round 73.
