In 2009, the Korean posted a career-best tie for second at the P&G Beauty NW Arkansas Championship presented by John Q. Hammons where she lost a three-way (with Angela Stanford) sudden-death playoff to Jiyai Shin; Yoo tied for third at the CVS/pharmacy LPGA Challenge, where she tied a career-low with a 64 in the second round; she finished sixth at the HSBC Women's Champions and tied for seventh at the Wegmans LPGA.

In 2010, Sun Young Yoo won her first LPGA event at the Sybase Match Play Championship having narrowly missed out on a LPGA victory the previous year. She won the Sybase Match Play Championship with a 3&1 victory over Angela Stanford. She also managed two other top 10 LPGA finishes when she tied for eighth at the LPGA Hana Bank Championship presented by SK telecom and tied for 10th at the Bell Micro LPGA Classic.

Yoo finished fourth at the 2011 HSBC Women's Champions; tied for seventh at the RICOH Women's British Open; tied for seventh at the RR Donnelley LPGA Founders Cup ; tied for 10th at the CN Canadian Women's Open; tied for 12th at the Kia Classic and tied for 15th at the Honda LPGA Thailand; tied for 20th at the Wegmans LPGA Championship; tied for 23rd at the Avnet LPGA Classic; tied for 25th at the US Women's Open; and tied for 28th at the LPGA State Farm Classic.

An impressive 2012 saw Yoo win her maiden major at The Kraft Nabisco championship. She made every cut of every tournament she played and Finished second in Greens in Regulation (75 percent).