Browne finished the tournament at 14-under-par 270 to earn his third career victory and first in six years. He ended with a one-stroke victory over Jason Bohn, who birdied 18 for a 3-under 68 in his final round at the TPC of Boston.
Browne, whose best finish this year had been a tie for 15th place at The International last month, won for the first time since the 1999 Colonial. He entered the tournament ranked 119th on the money list, but will make a big leap with a $990,000 paycheck.
'I'm speechless right now,' said Browne, who shared the overnight lead with Bohn, Pettersson, Billy Andrade and John Rollins. 'I really had to stay in the moment today and was really focused.'
Browne played the front nine at 3 under while some of the other co-overnight leaders struggled, but he really began taking control of the tournament around the turn.
The 46-year-old moved to 14 under with a 6-foot birdie putt at the 10th and saved par at the par-3 11th after knocking his tee shot off the back of the green.
Playing two groups behind Browne, Bohn sliced his tee shot into the woods at No. 9 and fell two strokes off the pace with a bogey that dropped him to minus-12.
Bohn stumbled to another bogey at the 10th hole, and Browne rolled in a par putt at 12 to remain at 14 under and move three strokes ahead.
But things would go less smoothly for the leader a few holes later.
Browne made an easy par at the 13th after hitting the flag stick with his 201-yard approach shot, and he parred 14 from the fringe. But the 21-year professional missed a short par putt at No. 15 after pitching within 4 feet from the rough, falling to minus-13 with his first bogey of the day.
Bohn, who won his only career title at this year's B.C. Open, then birdied the 14th to get to 12 under and close the gap to just one stroke.
But Browne all but clinched a victory with a birdie at the par-4 17th, where his gutsy approach shot locked onto the pin and bounced just past the hole to set up a makeable putt.
'I knew I was leading, but I didn't know by how much,' admitted Browne. 'I had a good feel with the shot that I hit, and I had a good feel with it all day long. So I went ahead and hit it, then managed to make the putt and it was great.'
Bohn strung together three straight pars after his birdie at No. 14 and needed to make eagle at the 18th to tie Browne. But his second shot landed just outside a greenside bunker, and the best he could manage was birdie.
Brandt Jobe and Fred Couples shared eighth place at 8 under after matching rounds of 69. Mark Wilson (69), Marco Dawson (71), Brett Wetterich (72), Bo Van Pelt (72) and Kent Jones (73) tied for 10th place one stroke further back.
First-round leader and unofficial host Tiger Woods shot a final-round 71 to finish in a tie for 40th place at minus-3.
'It was a frustrating last three days,' said Woods, whose charity is the main beneficiary of the tournament. 'I didn't have it, I didn't play particularly well, and consequently I was nowhere in contention.'
Andrade and Rollins both struggled to 4-over 75s in their final round. They ended up tied with 10 other golfers for 15th place at 6 under.