'I was extremely nervous,' said Steinhauer. 'I was doing my best to stay as calm as I could, and I am really proud of myself because I think I did do that.'
Kraft Nabisco Championship winner Grace Park shot a 68 to finish alone in second place at 10-under-par 274. Becky Morgan, who shared the 54-hole lead with Steinhauer, struggled to a round of 72 to take third place at 9-under-par 275.
Steinhauer was knotted with Morgan at minus-10 to start the final round and it was Morgan who jumped out of the gate early on with three birdies over her first four holes, to quickly break away at 13 under.
'I'm not going to worry about what Becky does,' said Steinhauer. 'I think it's actually tougher to get off to that kind of start and then maintain it.'
Trouble loomed for Morgan, who was seeking her maiden title on the LPGA Tour, and she dropped a shot with a bogey at the par-4 fifth. She countered with a birdie at the following hole but stumbled again with a bogey at the par-3 seventh.
Morgan hit a bad drive at the ninth and scrambled to a double bogey to let Steinhauer back in the tournament.
'I'm pretty disappointed,' said Morgan. 'I got off to a really good start. I had a few bad holes from the back of the front nine, I just couldn't get it back.'
Steinhauer took advantage of her playing partner's misfortunes and hit a 7-iron to 15 feet for a birdie at the par-4 10th. As Morgan fell further off the pace with a bogey at the 12th, Steinhauer kept plugging along with a tap-in birdie at the par-4 14th.
Steinhauer then added a birdie at the very next hole and cruised to her first title in almost five years despite a bogey at the 16th.
'If I've never been in this position I would never know what it felt like,' said Steinhauer, who also won this event in 1999. 'So I was able to come back on past experiences of being in the lead, or near the lead.'
Park ran off back-to-back birdies starting at the par-3 second and added a birdie at the sixth to move to 10 under. She faltered with a bogey at the seventh, but came back with a birdie at the 15th for her third runner-up finish of the season.
'I wish I could say I'm happy, but I'm not,' said Park. 'I still gave my best.'
Stacy Prammanasudh was penalized a stroke because of slow play at the par-3 16th. She shot a 68 to join Young Kim in a tie for fifth place at 5-under-par 279.