Grandmaison drank four beers and took two sleeping pills, before driving to a convenience store to buy lottery tickets and beer.
On his way home, he struck and killed Ngnan Wong Lee, 73, and struck her husband Poom Huy Shuy, 72, leaving him seriously injured.
Judge Marie-Josée Di Lallo said the combined effect of the alcohol and sleeping pills rendered Grandmaison impaired.
She said the man had ignored his doctor's warnings about mixing the pills with alcohol and ignored his pharmacist's directions to take just one tablet.
Grandmaison testified he took two pills because he was burned out and needed to sleep.
"Since the beginning, he maintained that it was just an accident," said Grandmaison's lawyer, Alexandre Paradis.
More than a year after the collision, Huy is still in hospital, said Crown prosecutor Dennis Galiatsatos.
Galiatsatos said he'll seek a long sentence in a federal prison.
"He is a repeat offender," the prosecutor said. "He caused the death of one person, and he's caused extremely grievous injuries to a second victim."
Huy's family declined to speak to reporters, but Galiatsatos said the man's relatives are happy with the judgment.
"They are kind of emotionally exhausted," said Galiatsatos. "They've been through a lot and they continue to be though a lot, because their father is still in the hospital over a year later."
Sentencing arguments as well as victim impact statements will be heard May 29.