The 27-year-old clocked 2:04.67 to take victory by 0.22s from Austria's Otmar Striedinger with Swiss skier Urs Kryenbuehl a further 0.05s behind.
It was a real day for the underdog in the French Alps as the sun made for faster conditions, particularly at the top of the course, for the later starters.
It was Cater's first podium finish with his best previous results sixth in the Kvitfjell Super G in February 2017 and in the Wengen combined last January.
He seems to have a preference for the longer tests with his best downhill showing before today an eighth place at Wengen in January 2019.
And, wearing start bib number 41, he gained time here with a great aerodynamic position, attacking the course and reaping his reward.
Striedinger matched his best result, a second in the Beaver Creek Super G back in 2013, while Kryenbuehl had just his second podium finish in 39 starts.
"I knew it was a good run but I didn't believe it was that good. I was hoping for top 10, top 15, just hoping not 36th or 37th or 50th because it's difficult sometimes. You think you're fast but you're not." - Martin Cater to FIS
While yesterday's Super G was beset by heavy snow and near-zero visibility, today the sun came out and the conditions were perfect.
But perfect conditions doesn't mean perfect skiing and there were plenty of thrills and spills on a dramatic day of downhill action.
Caviezel somehow shook off the pole, then the flag, and still led after four skiers.
For a while, it looked like reigning overall World Cup champion Aleksander Aamodt Kilde might claim his fifth career victory.
But Kryenbuehl pushed the Norwegian down to second before Striedinger, wearing bib 26, and Cater went even quicker.
There were a number of dramatic crashes with Swiss Niels Hintermann well in front on the clock before he lost a ski and ended up in the protective netting.
Canada's Cameron Alexander had to be stretchered away with what looked like a painful leg injury.
His team-mate Brodie Seger fell metres from the finish and had to be attended to in the finish area with Austria's Daniel Hemetsberger, who had already started, having his run yellow-flagged.
Hemetsberger finished just inside the points in 29th.
That wasn't the end of the drama for Team Canada as James Crawford, who looked set for a points finish, had to come out of his tuck position late on as a marshal strayed close to his racing line.
Crawford waved his fist in anger and ended up in 44th place.
But no doubt about the star of the day with Cater now hoping this is the start of something special.
He said, "Before I've skied well but couldn't bring it to the race. I hope it gives me some confidence for the next races."
His compatriot, two-time downhill world champion Ilka Stuhec, was quick to offer her congratulations.