It was an obvious indication of her potential as she joined the professional ranks in June 2003. She won two events on the LPGA Futures Tour - including her first pro event - and won her full card for the 2004 season. It was not all plain sailing, and after a sixth place during her rookie season and a runners-up place in 2005, her form dipped and she made only five cuts during 2006. She rebounded in 2007 to finish in the top 70 on the LPGA money list, and in 2008 it all slotted into place again as she finished the year in the world top 30.
In 2009, Hull's best finish was runner-up at the HSBC Women's Champions; tied for eighth at both the Kraft Nabisco Championship and the CVS/pharmacy LPGA Challenge; recorded her fourth international victory at the ANZ Ladies Masters on the Australian Ladies Professional Golf (ALPG) Tour.
Katherine Hull’s finest achievement of 2010 was when she won the Navistar LPGA Classic Presented by Monaco RV, finishing one shot ahead of Brittany Lincicome. She also came close to winning the RICOH Women's British Open but finished one shot behind Yani Tseng to finish as runner-up.
2012 saw Hull-Kirk record 3 top 10 finishes including a runner-up finish at the ShopRite LPGA Classic. She also made 21 cuts and finished the season ranked 66th in the world.