A jury convicted Francis Proulx in 2009 of killing Nancy Michaud in May 2008.
He was sentenced to life in prison.
His lawyers argued jurors were not sufficiently instructed about a possible verdict of not criminally responsible because of mental problems.
They also insisted Proulx didn't get a fair trial because his psychiatric expert witness admitted he had lied on the stand.
However, the appeals court ruled that had no impact on Proulx's case, pointing out that the witness was later found not guilty of perjury and had not meant to mislead the court.
The lawyers also asserted during the trial that Proulx kidnapped and killed Michaud because of medication he was taking.
Michaud, a mother of two, was an aide to then-natural resources minister Claude Bechard, who has since died of cancer.