Francis Proulx was found guilty in 2009 of murdering Nancy Michaud, who worked for Claude Bechard, a member of then-premier Jean Charest's Liberal cabinet.
Proulx was sentenced to life in prison without possibility of parole for 25 years.
He has acknowledged his 2008 crime, but argued he should not be held criminally responsible because of a mental disorder.
Proulx wanted the top court to overturn the conviction and order a new trial.
As is usual in leave to appeal rulings, the justices gave no reasons for refusing to hear the appeal.
The case was a grisly one.
Proulx entered Michaud's house in Riviere-Ouelle, in the lower St. Lawrence region, and took her hostage while her two children slept.
He also took credit cards and personal identification numbers and, shortly afterward, shot her in the head.
Before disposing of the victim, he had sex with her corpse.
During his trial, he argued he was not criminally responsible because of a mental issue, saying he was on medication at the time of the crime.
Although his lawyers asked the judge to explain the defence to the jury, the request was refused and no instructions were given.
The Quebec Court of Appeal refused last year to overturn the guilty verdict, saying the lack of instructions did not deny Proulx a fair trial.