Jean-Alix Miguel, a one-time kick-boxer, was sentenced to seven years in prison for manslaughter in 1990.
He successfully applied for a teaching post in 1998 after his release. But he was fired by Montreal's French-language school board in 2004 after it emerged that he had lied on his application by denying he had any convictions for violent crimes.
With support from his teachers' union, Mr Miguel challenged the decision in Quebec's high court. Last week it found in his favour, ruling that he should be reinstated and receive back pay to cover his time off work since 2004.
Mr Miguel had taken his case to Quebec's human rights commission, arguing that he could not be fired for a crime not linked to his position as a teacher.
Ginette L'Heureux, speaking for the commission, told CTV news in Montreal: "If you're a convicted paedophile, you can't get a job in a daycare. If you're an arsonist, you can't get a job as a fireman. But if the crime is not related to the job, you can't be disciminated against."
The high court ruled that the application question, which asked what past convictions he had unrelated to the job, was arbitrary and violated Mr Miguel's freedom to work, guaranteed by Quebec's charter of rights.
The court also accepted the commission's finding that Mr Miguel's position as a teacher was sufficiently different from the situation surrounding the killing of his girlfriend as to render that incident irrelevant.
The Montreal school board has announced it will appeal.