Guay managed the feat despite just getting back on snow for training sessions earlier this month due to knee surgery performed in the summer.
"Eighth is very respectable considering a month ago I wasn't really that confident whether I'd be here racing, so I'm very satisfied," Guay told Rob Snoek of CBC Sports.
Guay, who has landed on the World Cup podium 20 times in his career, booked a spot for Sochi based on having Top 12 finishes in the last two seasons.
The 32-year-old from Mont Tremblant said that qualifying in the season opener was relief.
"The monkey's off my back, I can now focus on what I have to do between now and February," said Guay, who finished in one minute, 50.98 seconds. "But it feels good to be the first one [from Canada] to qualify."
Dominik Paris took Saturday's race, his third career World Cup victory. The Italian crossed in a time of 1:49.90.
"It was very difficult, it's bumpy in the middle section and not so easy to get fast," said Paris. "I slid a little bit but tried my best to give it all at the finish."
Austrian veteran Kraus Kroell was just 3-100ths back of Paris in second, with Frenchman Adrian Theaux third. Aksel Lund Svindal of Norway, the two-time World Cup overall champion, finished fourth.
Bode Miller of the United States, making a return to the World Cup circuit after not competing last season, was 16th.
Canadian veterans Jan Hudec and Manuel Osborne-Paradis each earned points with top 30 results. Calgary's Hudec, who won the Lake Louise downhill in 2007, was 22nd, while North Vancouver's Osborne-Paradis was 28th.
Conrad Pridy of Whistler was 32nd, three spots ahead of Jeffrey Frisch, of Mont-Tremblant, Que. Ben Thomsen of Invermere, B.C., was 44th, one ahead of Morgan Pridy of Whistler.