The 31-year-old American fired a one-under-par 71 in the final round to finish 72 holes on 13-under 275 at the Earl Grey Golf Club in Calgary.
That was good enough for a two-stroke victory over Japan's Mao Saigo with South Korea's Ryu Hae-ran and Jenny Shin sharing fourth on 278.
World number 38 Coughlin managed her breakthrough triumph in her 101st LPGA start.
"It's kind of indescribable. Never really thought it would happen but it did," Coughlin said. "I've had people believe in me for a really long time and I wouldn't be here without their push and their love."
Coughlin's best career finishes so far this year had come at majors, a share of third at the Chevron Championship and fourth at the Evian Championship two weeks ago.
"I felt like I hung in there all day. I didn't really make anything, played super patient and stuck with it and they eventually went in."
"I don't give up and I think I showed that perfectly today."
Coughlin birdied the par-5 15th but Ryu matched her moments later to keep a one-stroke lead over her and Saigo.
But Ryu made three-putt bogeys at 16 and the par-3 17th and Saigo made a bogey at 17.
Coughlin meanwhile sank an eight-foot birdie putt at 17 to grab a two-stroke lead over Ryu and Saigo as she reached the 18th tee. Ryu suffered another bogey while Coughlin's routine par at 18 produced her long-sought title.
Saigo, 22, is a six-time Japan Tour winner who produced her best LPGA finish and fourth top-10 showing of the year.
World number 20 Ryu, whose only LPGA title came in last October's Northwest Arkansas Championship, was a runner-up at last week's Dana Open, her seventh top-10 effort of the season, and fifth at the Evian.
Ryu, who began the day with a one-stroke lead over Coughlin, stretched her edge with a birdie at the par-5 fourth and answered a bogey at seven with a birdie at the eighth.
Coughlin birdied the par-3 fifth but stumbled back with bogeys at the sixth and 10th holes, falling three adrift with eight holes remaining.
But Ryu sent her second shot into the water at the par-4 11th on her way to a double bogey and suddenly Coughlin was back within one.