Thompson, 18, won the LPGA's Lorena Ochoa Invitational by a stroke over Stacy Lewis, who is third in the world rankings. Fourth-ranked Ryu So-yeon was third while No. 1-ranked Inbee Park finished fourth and second-ranked Suzann Pettersen tied for fifth.
It was the fifth professional title overall for Thompson and moves her from 14th to No. 10 in the women's world golf rankings.
In September 2011 Thompson became the youngest -- 16 years, 7 months -- champion on the LPGA Tour when she won the Navistar LPGA Classic. That record stood just 11 months when Ko, at age 15 years, 8 months, won the CN Canadian Open.
This week Ko joins Thompson as an LPGA member. Her first tournament is to be the CME Group Titleholders beginning Thursday in Florida.
The race for year-end No. 1 will also be determined at the Titleholders. Park's ranking points average of 11.77 is just 0.4 points ahead of Pettersen with Lewis third at 10.03.
Park has been No. 1 for 31 weeks while Pettersen is looking to take over the top spot for the first time. Lewis was No. 1 for four week before Park took over in April.
The women's world golf rankings Top 10, listing player's name, home country and ranking points average:
1. Inbee Park, South Korea, 11.77
2. Suzann Pettersen, Norway, 11.37
3. Stacy Lewis, United States, 10.03
4. Ryu So-yeon, South Korea, 7.04
5. Lydia Ko, New Zealand, 6.87
6. Choi Na-yeon, South Korea, 6.58
7. Feng Shanshan, China, 6.18
8. Karrie Webb, Australia, 5.77
8. I.K. Kim, South Korea, 5.28
10. Lexi Thompson, United States, 5.21