Carin Koch, who held the lead over the first three rounds, managed a 1-over 73 to join Catriona Matthew in second place at 16-under-par 272.
Pak was three shots back to start the round and began her charge on the front side of the Crossings Course at Magnolia Grove Golf Club. At the par-5 fourth, Pak knocked a 5-iron within six feet for an eagle.
'After last year, I think I know how to play this golf course, especially since I won this tournament,' said Pak. 'I have a lot of confidence, so I played well again.'
More from Pak on her defense.
The 25-year-old hit a 9-iron to five feet at the par-4 ninth to tie Koch atop the leaderboard at 17-under. However, Pak ran into trouble with a bogey at the 12th to fall one shot behind.
At the par-3 14th, Pak began a remarkable stretch that propelled her into the lead for good. She landed her tee shot within eight feet of the cup and drained the putt for a birdie to regain a share of the lead.
'After the front nine it gets better and better,' said Pak. 'Of course I was putting really well, so I didn't have any problems on the greens.'
Pak took the lead outright with a two-foot birdie at the 15th. She reached the green in two and two-putted for birdie at the par-5 16th before sealing the deal with a 12-foot birdie at the very next hole to make it four in a row.
'I was thinking about maybe two or three in a row,' said Pak, who earned $122,000 for the win. 'That was pretty good in the end, a pretty good stretch. I'm really happy about it and I really enjoyed it.'
Pak won for the fourth time in 2002 and the 17th time in her career on the LPGA Tour
Koch, who opened the tournament with a 62 on Thursday, birdied the par-3 second, but found trouble with a bogey at the very next hole. She recovered with a 21-footer for birdie at the sixth, but bogeyed nine to make the turn at minus 17. Koch added a bogey at the 17th for a share of second.
'It's very disappointing,' said Koch. 'It's disappointing to shoot 1-over-par on Sunday of course. I didn't feel I played as bad as that score, but I didn't play great.'
Annika Sorenstam, who was looking for her 10th victory of the season, picked up three birdies on the front nine to make the turn at minus 16. She struggled with a bogey at the 12th, but responded with a birdie at the very next hole.
The Swede ran into trouble again with a bogey at the last for a round of 70. Sorenstam was joined by Wendy Ward and Rosie Jones at 15-under-par 273.
Final results from the LPGA Tournament of Champions