The 22-year-old timed 1min 10.82sec -- two tenths behind Italy's Manuela Moelgg in bright sunshine after weather had caused havoc with the schedule earlier this week.
"The toughest thing is mentally to tide yourself over until it is time to go," said race favourite Shiffrin, who won slalom gold in Sochi at just 18.
"Weve been race ready for the past five days and finally were racing today.
"I skied well, I was loose. But I also feel like I can go a little bit harder and theres nothing to hold back for on the second run."
Shiffrin, who has racked up 41 World Cup wins, was supposed to have completed her best two disciplines -- the slalom and giant slalom -- already.
But gusting winds that forced the postponements of the alpine skiing could have complicated her quest to medal in potentially five events.
Shiffrin, who stormed to gold in the most technical of events in 2014, a year after winning the first of three successive slalom world championships, could realistically target at least four gold medals in Pyeongchang.
- First run jitters -
"Its fair today," said the American as North Korea's famed "army of beauties" cheerleaders serenaded fans in the background while waiting for their skier, Kim Ryon Hyang, to go last.
"The conditions are great, the weather is beautiful and Im excited to see what I can do.
"I still think everyone has some first-run jitters so what you saw there wasn't a good depiction of what we can all do," she added.
"The second run is going to be like a new race so its going to be really interesting."
Tessa Worley, the French world champion seen as the biggest challenger to Shiffrin, had a nightmare, losing control at one point in a skid that will almost certainly lose her a shot at the gold medal.
"The tension got to me and I made that big mistake that cost me a lot of time," she said after her 14th place finish.
Moelgg, who went out first, was more than mindful of Shiffrin's threat.
"She is behind me -- here, here," the Italian told AFP, poking her heels with her ski sticks.
"Yeah shes always dangerous, every time. But Ill try to push and give my best. Its nice, no wind so its a fair race."
Meanwhile, Moelgg's fellow Italian Federica Brignone was third fastest in 1:10.91, just over half a second quicker than Norways Ragnhild Mowinckel with another Italian, Marta Bassino fifth.
Germany's Viktoria Rebensburg, who won gold in Vancouver and bronze in Sochi, was eighth fastest.
There was drama, meanwhile, when Switzerland's Lara Gut lost control and ploughed off the course, knocking over a group of photographers, who were scattered like tenpins.
"Its tough being a photographer," shrugged Gut.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)