Can a professional argue I contributed to my own losses in a negligence claim against them in Ontario?

Yes. A professional defending a negligence claim can argue that you contributed to your own loss - for example, by withholding relevant information, ignoring clear advice, missing a deadline you were told about, or failing to review documents you were given a fair opportunity to check. Ontario's Negligence Act allows a court to apportion responsibility between you and the professional as percentages, and your damages are then reduced by your own share of the fault, rather than the claim being defeated entirely.

This doesn't mean any client mistake automatically reduces recovery - the client's conduct has to have genuinely contributed to the loss and fallen below what a reasonable client would have done in the same situation, judged against what the professional actually communicated and when. A client who followed the professional's advice reasonably, even if it later proved wrong, generally isn't at fault for that. Because apportionment is fact-driven, records of what you were told, when, and how you responded - emails, signed instructions, meeting notes - tend to matter a great deal in how a court divides responsibility.