'It's a great feeling,' said Hjorth, who pocketed $195,000 for the win.
She posted a 5-under 67 on Sunday to finish the event at 14-under-par 274. It was Hjorth's third LPGA Tour win and first since 1999 when she won both the SAFECO Classic and the Mizuno Classic.
Prammanasudh managed a 1-under 71 for second at minus-13.
Ochoa, the overnight leader, was undone by a pair of three-putt bogeys at one and 18. She struggled to a 1-over 73 and tied for third place with Angela Park, who tied a new course record at The Senator Course with a 63 on Sunday.
Ochoa already won six times this year on tour and was in great shape to win her fourth consecutive start. The Women's British Open champion and Park came in at minus-12.
Hjorth began the final round four behind Ochoa, and it was her play on an early hole that made her a factor. Hjorth chipped in for eagle at the par-5 fifth and was suddenly within striking distance of the lead.
Both Hjorth and Prammanasudh birdied the par-5 eighth, but Hjorth took advantage of the next par-5. Hjorth had 5 feet for an eagle, but missed. She tapped in for birdie to tie for the lead, but Prammanasudh moved one ahead with a 15-foot birdie putt at No. 12.
Prammanasudh, one shot clear, missed a 6-footer for par at the 15th. That put her in a tie for the lead with Hjorth, who came up short on a relatively easy birdie putt at the same hole.
At the 17th, Hjorth holed a 25-foot birdie putt from off the green to move ahead of Prammanasudh, who missed a 20-footer that would have kept pace with Hjorth.
'There was still plenty left,' said Hjorth. 'Lorena and Stacy can both make birdies on the last. You never know.'
At the closing hole, Prammanasudh knocked her approach to 20 feet. Hjorth hit her second to 12 feet to apply the pressure to Prammanasudh and Ochoa, who got back within one thanks to a birdie at the 17th.
Ochoa three-putted for a bogey at the closing hole to leave it up to Prammanasudh. Her 20-footer did not fall, so she had to rely on Hjorth three-putting for any chance of a playoff.
Hjorth did not accommodate. She lagged her birdie effort to a foot and tapped in for her first win in eight years.
'I've been driving the ball so well all week,' said Hjorth, one of the longest hitters on the LPGA Tour. 'On a course like this, it's what it comes down to. You can take out some corners and save some pars.'
Karrie Webb shot a 4-under 68 to finish in fifth at minus-10. Amy Hung first shot a 63 on Sunday to set the course record and that got her into sixth at 9-under 279.
Hye Jung Choi (68) and Angela Stanford (72) tied for seventh at minus-8. Teresa Lu (69) and Nicole Castrale (72) shared ninth place at 7-under-par 281.