Charlotte Kalla of Sweden took silver, more than 20 seconds adrift, while Marit Bjoergen of Norway and Finn Krista Parmakoski dead-heated for the bronze medal.
Kalla, who won the women's skiathlon on Saturday, began the race as odds-on favourite for a second Pyeongchang gold and started powerfully, immediately taking the lead from Bjoergen at the first split time.
The 30-year-old Swede looked on course to win her country's third women's cross-country gold of the Games, but Haga, ninth in the overall World Cup standings, had other plans.
She accelerated away on the second lap, building up a huge lead over Bjoergen as she passed the 8.4km split and maintained her blistering pace all the way to the finish line.
"When I started the last downhill, I was thinking that 'This is the most important downhill of your life. You better stay on your feet.' I gave it all I had because I feared that Kalla would go even faster in the end of her race," Haga said.
"The way that I grabbed this opportunity... wow, I am truly satisfied with myself."
Kalla, who won gold in the event in Vancouver in 2010, tried to raise her tempo but simply did not have enough in the tank to close the gap and had to settle for second place.
DIGGINS MISSES OUT
Parmakoski also put in a strong burst towards the end, delighting the noisy Finnish fans in the crowd as she finished on exactly the same time as Bjoergen to claim a bronze medal.
Jessica Diggins came close to winning a first cross-country medal for the United States since 1976 but the Minnesota native came up agonisingly short, finishing 3.3 seconds behind the bronze medallists in fifth place.
For Haga, it was a case of her hard work paying off on the Olympic stage.
"I have practiced the 10km free a lot this year and I got a feeling that I had a good start in terms of speed. I know I have to do better than I have previously to follow the others from the start," she explained.
"I got feedback during the race that said I had not dropped too many seconds and I had not used too much energy, and I felt it could be a good one. But I was frightened of Kalla on the last lap."
With her namesake Ragnhild Mowinckel winning silver in the women's giant slalom -- Norway's first women's Alpine medal since 1936 -- it was a good day for a traditional Nordic name.
"It is wild. When (she) won silver, we decided that today is Ragnhild Day. That was really inspiring," Haga said.
(Reporting by Philip O'Connor; Editing by John O'Brien)
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