ÉVIAN-LES-BAINS, France — With two holes left to play at the Evian Championship on Sunday, Lydia Ko stayed as relaxed and composed as a seasoned veteran. Ko, a South Korea-born New Zealander, birdied both holes on the shores of Lake Geneva to reach a new landmark in her short but already storied career.

With a flawless final-round display that featured spectacular approaches and clinical putts among her eight birdies, Ko became the youngest major champion in L.P.G.A. Tour history. She closed with an eight-under-par 63 for a six-stroke victory over Lexi Thompson at the final major of the season.

It was the best round of this year’s tournament, a score that helped her to a ninth career victory with a 16-under total of 268.

“To say that I’m the youngest in history for now, it’s so cool,” Ko said. “To finish with two birdies, finish on the last, 72nd hole with a birdie — it doesn’t happen often, so it’s definitely one of the top rounds of my whole entire life, and I’m sure it will be in my career.”