The 39-year-old tour veteran and Queensland native was hopeful when she arrived for the final 18.
"I feel very fortunate to have won today but I liked my chances at the start of the day because of the wind picking up," Webb said.
"If we had another day like we had the first three days, I probably was a little too far back to have a chance, so I was thankful for the weather changing and I played as good as I have in a very long time."
Beginning her round five shots off the lead, Webb carded six birdies against two bogeys to post a 12-under total of 276. Then all she could do was wait, which turned out to be a rather long wait since 54-hole leader Chella Choi was on the 14th hole. When Choi missed a 7-foot birdie putt on the final green an hour or so later, Webb could exhale and celebrate her 40th win.
"I got off to a great start and I just did a great job mentally today," she said. "I can't think of another time when I held myself together as well as that."
It was quite a turnaround after Webb's disqualification for signing an incorrect scorecard at the Australian Ladies Masters.
"It's amazing what happens," she said, "what a difference a week makes. Obviously, this time last week I wouldn't be expecting to be sitting here, so I'm glad things changed around quickly for me."
Webb moved up one spot to No. 7 in the Rolex Rankings.
Bubba Watson shot 64-64 on the weekend at Riviera Country Club to win the Northern Trust Open, his first PGA Tour title since slipping into the Green Jacket at the 2012 Masters.