Webb played the final 10 holes in 7 under at Dundonald Links. The 42-year-old Australian star birdied the par-4 ninth, made five straight birdies on Nos. 11-15 and added another birdie on the par-5 18th.
Kerr closed with a birdie on No. 9 in her bogey-free round in the event sanctioned by the LPGA for the first time. It has been part of the Ladies European Tour since 1986.
Fellow major champions Stacy Lewis and Inbee Park were tied for third at 69 along with Sei Young Kim, Lina Boqvist and Pornanong Phatlum. ANA Inspiration winner So Yeon Ryu had a 71, and Michelle Wie shot 72.
Lydia Ko opened with a 74. She has gone a full year without an LPGA win.
The Ricoh Women's British Open is next week at Kingsbarns.