With the slalom part of the event to come later, Mancuso put 1.
04 seconds between herself and her German rival for the first women’s Alpine skiing title at the Sochi Games, slicing down a sunlit Rosa Khutor in 1:42.68 to top the leaderboard.
Hoefl-Riesch, who beat Mancuso to gold in Vancouver four years ago, was only fifth quickest after the downhill leg.
The tall German will still be hopeful of charging for gold later in the slalom leg, having had nine career World Cup wins in the technical discipline which is not Mancuso’s strength.
She will have her work cut out, however, with the likes of Swiss Lara Gut, who was second 0.47 seconds behind Mancuso, and Slovenian all-rounder Tina Maze (0.86) also in front of her after the downhill.
“It wasn’t perfect but a lot can still happen in the slalom,” a relaxed-looking Hoefl-Riesch told reporters.
The experienced Mancuso, Olympic gold medallist in giant slalom in Turin in 2006, celebrated as if she had already won the title in the finish area where her grandmother was watching.
“It felt good. I was definitely looking for extra speed when I kicked out of the start gate,” she told reporters.
“I was really happy to cross the finish line and see I was in the lead. But that other run is pretty tough and it’s not my strength. But I’m really going to try my best.
“Anything is possible at the Olympics, you can never count anyone out.
“I already said I’m doing it for my grandfather, who can’t be here this year. But my grandma’s here,” the 29-year-old added.
Mancuso has not raced a World Cup slalom this season and has never been on the podium but fellow American Laurenne Ross said there was hope for her team mate.
“It’s going to be pretty tough,” Ross, who crashed out after losing a ski, told reporters.
“But it helps that it’s sunny and a little warm. I think that it’s actually going to be something that Julia looks forward to and she always manages to pull out a pretty good slalom run. I’m excited to watch and see what she can do.”
Liechtenstein’s Tina Weirather, one of the leading racers on the World Cup this season, did not start but hopes to have recovered from a leg injury in time to compete in Wednesday’s downhill.
(Additional reporting by Mark Trevelyan; Editing by Peter Rutherford)