Can a will be challenged if the person lacked mental capacity when they signed it in Ontario?

Yes. For a will to be valid in Ontario, the testator must have had "testamentary capacity" at the time of signing. This means they must have understood the nature of what they were doing (making a will), the extent of their property, who their natural heirs are, and how the will disposes of their estate. A diagnosis of dementia or another cognitive condition does not automatically invalidate a will — the question is capacity at the specific moment of signing.

A will can be challenged on the grounds of lack of testamentary capacity or undue influence (where someone pressured the testator into making the will). If a court finds the will invalid for either reason, the estate is treated as if the person died without a valid will and the intestacy rules apply.

These challenges can be costly and emotionally difficult for families. If there is reason to believe someone's capacity may be in question, having a doctor assess capacity at the time of signing and leaving notes in the file is advisable. A lawyer who witnesses the signing can also attest to their observations.