'I definitely had my bad round yesterday,' said Lunke, who struggled home with a four-over 75 in the final round. 'I just tried to play my game today, to play stroke-play and I'm thrilled.'
Lunke won the title with a one-under 70 on Monday, followed by Stanford's 71 and a two-over 73 by Robbins, whose chances fell with a double-bogey six at the 13th hole.
For the second consecutive day, it all came down to the play at the par-five 18th at the Witch Hollow Course at Pumpkin Ridge. On Sunday, Annika Sorenstam bogeyed the hole to miss the playoff by a stroke, but Stanford drained a long birdie putt to join the 18-hole playoff on Monday.
Lunke took a one-shot lead over Stanford to the 90th hole of the championship and both hit the fairway off the tee, but Lunke laid up in the fairway while Stanford's second finished in the left rough.
Lunke pitched 12 feet short of the hole while Stanford, in a horrible lie, ran her approach short of the green on the fringe. Stanford once again sank a magical putt at the closing hole as her 25-footer fell into the bottom of the cup, tying her with Lunke atop the leaderboard at even-par.
'When it disappeared, I thought, 'I can't believe that happened again,' said Stanford, who earned her first LPGA Tour victory last Sunday at the ShopRite LPGA Classic. 'I didn't get my hopes up too high.'
Lunke, indeed, stepped up and ran home the curling birdie putt of her own to win her first major championship.
'I knew Angela was going to make it,' said Lunke, who pocketed $560,000 for the win. 'I was just trying to focus on my line. I pretended like it was match-play and your opponent is always going to make it.'
Not only was this Lunke's first major title, it was her first victory on the LPGA Tour. She became the 26th player to make win No. 1 a major and the first since Se Ri Pak took home the LPGA Championship in 1998.
Lunke became the first player to win the trophy by qualifying for the event. Even that looked to be in jeopardy when she opened with an 80 in the opening round of sectional qualifying, but now, at 24, she is the youngest American to win a major title in the last 16 years.
'I hope it's the start of something big,' said Lunke. 'I'm not going into any event differently now than I would have before, but I'll have it in the back of my mind that I really can do it.'
Lunke had the lead for most of the playoff and Stanford battled back from a terrible start with back-to-back birdies on 11 and 12. Then at the par-four 14th, Stanford's caddie urged her to hit seven-iron and she did, but the ball bounced through the green. Stanford chipped in for birdie to finally match Lunke in first at even-par.
'I was all over the place on the front nine,' said Stanford. 'After the chip- in at 14, I felt like the momentum was going my way. I felt like I could take this if I want it. It's unfortunate. I wanted to win, but it was so much fun.'
Both Lunke and Stanford parred the 15th and 16th holes, but a wayward approach at 17 cost Stanford. Her second landed in a greenside bunker and she blasted out to six feet. Stanford's par save died right around the hole and the bogey dropped her one back as she made the move to 18.
The three finished tied at one-under-par 283 after 72 holes and returned to the course Monday morning for the first 18-hole playoff at the U.S. Women's Open since 1998 and the first three-way playoff since 1987.
Lunke's short game carried her early, as she missed the green at No. 1. Her pitch left her a foot from the hole, where she tapped in for par, but her adversaries did not have the same fortune. Both Robbins and Stanford landed in a greenside bunker and made bogey to fall one behind.
Stanford continued to battle her swing at the second when she missed the green short and chipped 12 feet from the hole. Her par-saving putt flew by the hole so it was back-to-back bogeys and a two-shot deficit behind Lunke, who parred two after once again missing the green.
Robbins bogeyed three and Lunke birdied four to give Lunke a four-shot lead over Robbins and a three-shot edge over Stanford. At the sixth, Lunke holed a 20-foot birdie putt after hitting her first green in regulation and Robbins sank a seven-footer for birdie to keep pace with Lunke.
One hole later, Lunke drove into 'the worst lie I've ever had' and could only advance the ball a few feet. She hit her third to where the others lied in two and was not able to save par. Robbins hit her third to 20 feet and converted the birdie try to get within two of the lead.
Stanford's woes continued at eight when she pulled her approach into the gallery and left a 12-foot par save on the edge of the hole. When all three made the turn, Lunke was two ahead of Robbins and four up on Stanford.
Robbins birdied the 10th and looked poised to leave the green tied for the lead, but Lunke holed an eight-footer for par after finding a bunker off the tee.
Robbins bogeyed the 11th and Stanford birdied both 11 and 12 to get back into the hunt. Lunke made a mess of the 12th hole, missing the green right, then chipping through the green before leaving with bogey and a slim lead over both Robbins and Stanford.
At the 13th, Robbins hit into a bunker and chunked her third shot. She missed a five-footer to post a double-bogey and essentially shoot herself out of the tournament.
'It's unpredictable out there with what's going to happen,' said Robbins, who is winless on tour since 1999. 'Hitting the one poor shot and now I was pressing a little bit. I was trying to give myself chances.'
Lunke made the most of her chances, as she hit only eight greens in regulation but needed only 23 putts in the playoff. She had finished no better than 15th in her LPGA career.