Her victory put an end to 2022 champion Jakara Anthony's bid to become the first Australian to successfully defend a title at a Winter Games.
Anthony, who has dominated the discipline for the last four years, led from silver medallist American Jaelin Kauf after the first run but wobbled out of line on her second to finish eighth and last.
Kauf, who also finished second in Beijing four years ago, won her second silver with a score of 80.77, and Pyeongchang 2018 champion France's Perrine Laffont clinched bronze with 78 points.
Control and style
In her first run of the finals, under a bright and sunny afternoon sky in the narrow valley of Livigno, Lemley was in firm control of her skis, always pinned one to the other, and landed well after two big jumps.
In the second run - also known as the superfinal - the skier pushed on the speed, keeping her skis tight together and showing off great technique around the moguls and with a big grab on the bottom jump.
"I was trying to focus on trusting myself. I've been working on that this whole season... and right when I pushed out of the gate, I said to myself 'Let's go!'" Lemley told a press conference after the medal ceremony.
Like other athletes at the Winter Games, Lemley's medal fell apart as she was celebrating on the top of the podium just after she received it from Australian Olympic Committee President Ian Chesterman, who would have been hoping for a different outcome.
Anthony had a first fast, clean run for the finals, with smooth turns and a big grab on the bottom jump, but in the second she lost control in the middle section of the moguls, leaving her fall line, the straightest path down the hill.
Her 83.96 score on the first run would have been enough for gold had she reproduced it in the second, but only the final score counts and Lemley's 82.30 secured the American her first global title.
Not the easy route
Kauf, who got through to the final only hours before in the second qualifying round, delivered a fast first run down the hill, with tidy skiing through the moguls and strong jumps.
In the superfinal she was strong and quick, with a big top jump, crushing through the moguls and performing a grab in the bottom air.
"I definitely didn't take the easy route into the finals... but I think everything happens for a reason," Kauf said.
Despite scraping through to the second final round with a stylish technique but not so clean jumps, Laffont set a very high bar in the second run, showing off great precision and performing two big jumps with grabs.
Japan's Hinako Tomitaka also scored 78 but Laffont took the bronze as she was awarded a better score - 42.6 to 42 - for her turns in the final run.
Asked what went through her mind when she saw her score was the same as Tomitaka's, Laffont said: "Today was a real roller coaster. It is pretty hard to remember what I was thinking."
Anthony was disappointed but was already looking ahead.
"I'm pretty bummed, to be honest," Anthony said. "I definitely had what it took, but it is just not my day today sadly. Maybe in another four years' time."