Trailing by two shots after round 1, Webb carded a second-round 64 to take the lead and followed that with rounds of 70-69 for a 270 total. Diaz, who trailed Webb by seven shots after round 2, shot 66-68 over the weekend to finish alone in second. First-round leader Wendy Ward tied for third with Maria Hjorth.
Webb and Mi Hyun Kim were the only players in the field to post four sub-par rounds. Kim finished alone in eighth.
At the age of 26, Webb joined Mickey Wright, Louise Suggs, Pat Bradley and Juli Inkster as the only players to win four different majors in their LPGA careers. Webb will be trying to join Inkster, Patty Sheehan and Wright as the only players to win this event two straight years.
Webb almost withdrew from the last year's tournament Sunday morning because her grandfather suffered a stroke a few days prior. She had booked a flight to leave with her parents Sunday afternoon but her father spoke with the family and they thought that her grandfather would want her to stay.
This is the second major on the LPGA Tour calendar this season. Annika Sorenstam captured the season's first major, the Kraft Nabisco Championship back in March.
This is the ninth straight year that the McDonald's LPGA Championship is being held at this course. Prior to DuPont Country Club, the event was held at Bethesda Country Club from 1990-93 and Jack Nicklaus Golf Club from 1978-89. Prior to that, the event was held at numerous venues.
Before changing into the McDonald's LPGA Championship in 1994, the tournament that was held at this site was known as the McDonald's Classic (1981-82), McDonald's Kids Classic (1983-84) and McDonald's Championship (1985-93).
When Se Ri Pak captured the title in 1998, she became the only wire-to-wire winner of the event.
Check out the field for this week's McDonald's LPGA Championship