Jennifer Johnson (72), Haeji Kang (69), Anna Nordqvist (70) and Gerina Piller (70) tied for third at 9 under.
Johnson opened with a course-record 62 and followed with 70 for a spot in Sunday's final pairing with Lewis and Kim. But the 22-year-old Californian had a double bogey and two bogeys on the back nine to fall out of contention.
Kim had a run of three straight birdies on No. 9-11, but shot 3 over on the final seven holes, including a double bogey on 18.
"I hadn't been in contention in a while so I kind of forgot what it was like having nerves," Kim said. "And it kind of showed on the last hole."
Lewis smiled and pumped her fist to the crowd as she walked down the fairway at 18, relishing her new place atop the world ranking.
Her brief stay as No. 1 last year was a rocky one, with Lewis admitting that she had trouble dealing with the extra obligations that came with the top spot.
"With a good team of people around me," Lewis believes she's more prepared to handle those duties and be the face of the LPGA Tour.
"The last time it was taken away from me in an off-week when we weren't even playing, so I'm definitely just not going to take it for granted and really enjoy it this time. Now I know all the extra things that come along with it. But I'm ready for it this time."
Karrie Webb, last year's champion, tied for eighth after a 67. Third-ranked Lydia Ko bounced back from a second-round 75 to shoot 69 and finish at 1 under.