The 40-year-old American, who last raced at this level at the world championships in Are in 2019, was 31st to start and clocked 1 minute 16.36 seconds, 1.18sec behind the winner Cornelia Huetter.
"Today was the perfect start. I had a solid run, got to the finish," said Vonn who was given a wild card to compete at St Moritz.
"The first one is the hardest. I felt really good. The course was a little bit bumpy when I went, so in some sections I was a little more conservative with my lines.
"But in some sections, I was really fast. There's definitely a lot that I have left to give, I just wanted to get to the finish today and be solid."
Austrian Huetter was 0.18sec ahead of Swiss skier Lara Gut-Behrami with the Italian Sofia Goggia completing the podium.
Huetter moves second in the overall standings, 37 points behind Camille Rast of Switzerland.
She is third in the discipline standings 10 points behind joint leaders Goggia, who won the opening super-G of the season at Beaver Creek last week, and Gut-Behrami.
'Stressed'
All eyes, however, were on Vonn who has come back to the sport hoping to qualify for the 2026 Winter Olympics.
"I was a little bit stressed," she admitted after receiving a standing ovation on the finish line.
"I was a little bit behind. I forgot a couple of things. It was hectic, I just hadn't done it in a while but it was nice to feel the nerves again.
"It feels so good to have the nerves and to have the butterflies, the adrenaline to push yourself.
"I always feel it’s just me against the mountain. I love that feeling."
Vonn left the sport in 2019 after a series of major injuries but she has been free of pain since a right knee replacement in April, when part of the bone was cut off and replaced by two titanium pieces.
She was the most decorated women's skier with 82 World Cup victories when she retired. Her mark has since been eclipsed by fellow American Mikaela Shiffrin's 99.
Vonn, the 2010 Olympic downhill champion, also won 20 World Cup titles, including four overall crystal globes, eight world championship medals and secured 137 World Cup podiums.
She will continue her return with another super-G at St Moritz on Sunday.
"Tomorrow will be better now that I'm used to it," said Vonn.
