In the playoff, Webb and Ochoa were both over the green at the par-5 18th in two. Webb chipped her third shot to seven feet. Ochoa, who led since her opening round 62, rolled her chip 18 feet past the cup.
The Australian fired the low round of the day Sunday as she posted a 7-under 65 to finish at 9-under-par 279. Ochoa, who held the lead after each of the first three rounds, eagled the last to shoot 72 and force the playoff.
'I really made some good putts on the front nine and hung tough on the back,' said Webb, who claimed $270,000 for the win. 'I had plenty of chances. I'm glad I didn't have to make a putt on the last, just put it in the hole.'
Michelle Wie held the lead late on the back nine, but could only par her final two holes to end one stroke back at minus-8. She shared third place with Natalie Gulbis, who closed with a 68 on the Dinah Shore Course at Mission Hills Country Club.
Two-time champion Juli Inkster also shot 68 to end alone in fifth place at 4-under-par 284. Last year's winner, Annika Sorenstam, tied for sixth place at 2-under-par 286 after a 70. She was joined there by Hee-Won Han (71).
Webb, who never had the outright lead until the 72nd hole, began the round seven strokes behind Ochoa. Webb steadily climbed the leaderboard on the front nine.
The LPGA Tour Hall of Famer birdied the third for the second day in a row to move to minus-3. Webb came back with a birdie on the par-3 fifth. The Australian collected the third birdie of her opening nine on the seventh.
Webb continued to make her move. She poured in a birdie putt on the par-4 10th and made it two in a row with a birdie on 11. Webb parred her next six holes to remain in contention at 7 under.
The 31-year-old found the left rough off the tee on the 18th. She played short of the water fronting the green with her second. Webb's pitching wedge from 116 yards out bounced twice then rolled into the cup for eagle.
That jumped her to minus-9 and the outright lead. Then it was off to the driving range to wait and see what would happen.
'It was a perfect number, 116 with a little breeze in my face. It was just a perfect wedge and I hit it straight at the pin,' said Webb. 'I knew I needed to make birdie and when it was in the air, I wanted it to just get close. Then it went in and I think I nearly had a heart attack when that happened.'
Ochoa, who is still looking for her first major title, opened with a birdie on the second, but struggled badly from there. The Mexican faltered to four bogeys in a seven-hole stretch from the ninth to tumble out of the lead.
At the 16th tee, she stood at minus-6 and was behind Webb, Wie and Gulbis. Ochoa got one stroke back with a 9-foot birdie putt on the 16th. At the last, Ochoa's second shot rolled within 13 feet of the cup and she sank that putt for eagle to force the playoff.
Wie came flying out of the gate with birdies on one and two. She tripped to a bogey on the sixth and trailed by three. The 16-year-old came right back with birdies on eight and ninth to tie Ochoa in the lead. Wie tripped to bogeys on 12 and 14, but still owned the lead thanks to Ochoa's struggles.
The young Hawaiian got one stroke back with a tap-in birdie on 16 after her approach shot stopped within inches of the cup. Moments after her second shot, Webb holed her eagle at the last to take the lead.
Despite her birdie on 16, Wie now trailed by one. She two-putted for par on 17. With a chance to win the tournament outright with an eagle, Wie knocked her second shot to the par-5 closing hole over the green.
Wie, needing to hole her chip to win, ran her third some 9 feet by the hole. She now needed to make that to get into the playoff, but her birdie putt slid by the left edge. So, Wie shared third with Gulbis.
'It is kind of disappointing to end one shot short, but I played very solid out there and I did the best I could,' Wie said. 'I tried my hardest. I think Lorena and Karrie played very well and they deserved it.'
Gulbis, who is winless on the LPGA, was right there all day. She bogeyed the first, but came right back to birdie No. 2. Gulbis birdied the sixth to get to 5 under.
The 23-year-old ran off six straight pars around the turn. Gulbis grabbed a share of the lead with birdies on 13 and 15. She got to 8 under with a birdie on the 17th. Gulbis played short of the water on 18, then missed her birdie putt at the last that could have gotten her into the playoff.
'We heard on 16 that the bar was set at minus-nine and we had two holes left, so we were like, 'OK, game on,'' Gulbis said. 'I didn't have anything to lose today so I was just trying to make birdies.'