The big Norwegian would have been confident of taking the title in St Moritz after a breathtaking run that showcased his trademark aggressive style.
But Jansrud could only give a resigned shrug as Guay crossed the line 0.45 seconds quicker to take top spot in the Swiss resort.
Guay’s gold is the second World Championship title of his lengthy career, following on from his triumph in the Downhill at Garmisch-Partenkirchen in 2011.
The 35-year-old has been competing at the top level since 2003, but has not been at his best for a number of years, last finishing on top of a World Cu podium in 2014.
Guay phoned up the mountain to give compatriot Manuel Osborne-Paradis some advice on racing line, and the result was two Canadians on the podium, with Osborne-Paradis picking up bronze.
“It’s incredible,” said a clearly awestruck Guay after the race.
“With the whole history with all of my injuries and even the crash last week in Garmish, it’s just amazing to be standing here as a world champion.
“I’m having a hard time finding words to express what this means and how I feel. To be on the podium today with two of my good friends Manny and Kjetil makes it even better.”