Can improper service be a reason to set aside a default judgment in Ontario?

Yes — if a default judgment was obtained against a defendant who was never properly served, that's a strong basis for asking the court to set the judgment aside. Since default judgment assumes the defendant knew about the lawsuit and simply chose not to respond, a genuine failure of service undermines the fairness of the whole process: the defendant may have had no real opportunity to defend themselves at all. A motion to set aside on this basis typically focuses on demonstrating that the method of service used didn't actually comply with the applicable rules, or that the defendant genuinely never received notice despite the plaintiff's efforts.

This is somewhat different from the more general test for setting aside a default judgment where service was proper but the defendant simply missed the deadline, which weighs things like the reason for the delay and whether there's an arguable defence. Where service itself was defective, courts tend to treat the fairness concern as more fundamental, since the entire basis for treating the defendant as having had their chance is undermined. That said, a defendant relying on this argument still generally needs to act promptly once they learn about the judgment, and should be prepared to explain both the service problem and, generally, what their defence to the underlying claim would be.