Can my employer rely on misconduct discovered after firing me to justify the dismissal in Ontario?

Yes. Ontario law generally allows an employer to rely on serious misconduct discovered after the dismissal already happened, even if that was not the reason given at the time, as long as the misconduct existed and was serious enough to justify cause on its own. This is called after-acquired cause, and it means an employer is not necessarily stuck with only the reasons it knew about or stated at the time of termination if it later uncovers something that independently would have justified dismissal without notice.

The misconduct discovered afterward still has to meet the same high bar for just cause as anything discovered before the dismissal - it cannot be a minor issue used retroactively to avoid paying notice that was otherwise properly owed to the employee. This doctrine is most relevant in wrongful dismissal litigation, where new evidence sometimes surfaces during the legal process itself, such as through document disclosure, and it can significantly change the value of a claim if it genuinely applies to the facts uncovered.