On the clock for slow play and down a shot to Brittany Lang with three holes to go, Pettersen drained birdie putts on holes 16 and 17, before a par on 18 to win the LPGA Classic by one stroke at 22-under par at Whistle Bear Golf Club.
Pettersen's 3-under par 69 was her highest score of the four rounds and she still managed to hold off Lang, who was 7-under par 65 on the day and finished at 21-under.
"I felt I played some really good golf this week," Pettersen said, despite not being able to match her three bogey-free rounds. Her previous best finish at the Manulife Classic was a tie for sixth in 2013.
"I was pretty flawless through three rounds and broke that effort pretty quickly with a couple of bogeys, but I managed to get my head together and play some good golf. I played aggressive and it was a good day on the golf course."
Down two shots from where she started the day after five holes, it was a birdie on the eighth hole that got Pettersen going.
"I kind of felt that set the pace a little bit," said Pettersen. "Then I eagled 12 and that was a bit of a momentum changer. I knew the scores were not as low as they had been earlier in the week. I was still trying to get to a number and I didn't get there, but it was good enough.
"I thought I had much more of cushion. Brittany finished strong."
Lang had a one-shot lead when she walked off the golf course, scoring eight birdies on the day and one bogey, with three of those birdies coming on holes 15 to 17 to overtake Pettersen. While Lang was on the putting green in case of a playoff, Pettersen came storming back for the victory.
"I'm going to take nothing but positives away from this week," Lang said. "It was fantastic golf with three majors coming up. But the competition out here, it's unbelievable. It's really hard to win nowadays and it's great, it's great for women's golf. The girls are just, there's so much talent, so much depth."
Both Mariajo Uribe and Cristie Kerr, who were right behind Pettersen after the third round, faded in the final 18 holes. Uribe, from Colombia, was even par on the day and finished in third place at 18-under, while Kerr shot 1-over par 73 for 16-under par and ended in a four-way tie of fourth place with Minjee Lee, Jacqui Concolino and So Yeon Ryu.
The win is Pettersen's first on the tour since the Sunrise Taiwan LPGA Championship in October 2013 and the fifth time in a row she's won when leading after 36 holes of a tournament. But it's been a long time coming, the Norwegian said.
Hamilton's Alena Sharp had an up-and-down day with three birdies and four bogeys to finish 1-over par for the round and 10-under for the tournament and in a tie for 27th place.
"I had a couple three putts otherwise I would have been under par. I three-putted eight and nine, so there's 1-under right there," Sharp said, adding that the wind that everyone kept talking about finally showed up today.
"I couldn't believe it. Driving into today I'm like, oh, it's going to be like the other days, but by the time we hit, even number three it started on up for us, but really started picking up on seven. I think this is the way it's supposed to be playing, normally it's windy like this. So it's tough."
Sharp finished as low Canadian for the third time in the last four years at the Manulife Classic, also accomplishing the feat in 2012 and 2013. It is also the lowest ever finish by a Canadian, beating the previous low of a tie for 30th place.
Toronto's Sue Kim shot just 1-under par on the day and 5-under for the tournament. Sara-Maude Juneau of Fossambault-Sur-Le-Lac, Que., carded a 1-over par for a 4-under par total, and Natalie Gleadall of Stratford, Ont., struggled to a 6-over par to finish even par in her first Manulife Classic.