The general rule is that a contract made by person who is mentally ill is voidable by the person when she regains her sanity, or, as appropriate, by a guardian. If, though, a guardian has been legally appointed for a person who is mentally ill, any contract made by the mentally ill person is void, but may nevertheless be ratified by the ward (the incompetent person who is under a guardianship) upon regaining sanity or by the guardian.1
However, if the contract was for a necessity, the other party may have a valid claim against the estate of the one who is mentally ill in order to prevent unjust enrichment. In other cases, whether a court will enforce a contract made with a person who is mentally ill depends on the circumstances. Only if the mental illness impairs the competence of the person in the particular transaction can the contract be avoided; the test is whether the person understood the nature of the business at hand. Upon avoidance, the mentally ill person must return any property in her possession. And if the contract was fair and the other party had no knowledge of the mental illness, the court has the power to order other relief.