Kerr was born in Miami, Florida, and started playing golf at the age of eight. She had a successful amateur career, winning the 1994 Junior Orange Bowl International Golf Championship and the 1995 Women's Western Amateur. She was the 1995 American Junior Golf Association Junior Player of the Year. In 1996 she played in the Curtis Cup and was the low amateur at the U.S. Women's Open. She graduated from Miami Sunset High School in West Kendall, Florida.
She turned professional in 1996 at age 18 after graduating from high school, playing on both the Futures Tour and Players West Tour. In October 1996, she tied for sixth at the LPGA Final Qualifying Tournament to gain exempt status for 1997. Her LPGA career started fairly slowly; she broke into the top fifty on the money list in her third season in 1999, but did not win until her sixth season. In 2002, she won her first LPGA event at the Longs Drugs Challenge in California. By 2004, she was one of the leading players on the tour, with three tournament victories, and a fifth-place finish on the money list. She won two tournaments in 2005 and moved up to third on the money list. She tied for second at the 2000 U.S. Women's Open, matched by her performance in the 2006 Women's British Open. Her first win of 2006 came at the Franklin American Mortgage Championship where she posted a tournament-record score of 19 under par. In 2006, she was the only American to win more than one event on the LPGA Tour, winning three times. She won the 2007 U.S. Women's Open, her first major championship. She was also a member of the United States Solheim Cup team in 2002, 2003, and 2005.