Can someone challenge who was appointed as estate trustee in Ontario?

Yes. In Ontario, if you believe the wrong person has been appointed as estate trustee (executor), or that a person who has been appointed should not be acting, you can bring an application to the Superior Court to challenge the appointment or to have the estate trustee removed.

If probate has not yet been issued, you can file a caveat to pause the process while your objection is heard. If probate has already been granted, you can bring a court application to revoke the Certificate of Appointment of Estate Trustee on the grounds that it was improperly obtained — for example, because the will itself is invalid, because there was a more recent will that was not disclosed, or because the person who applied was not entitled to do so.

Courts can also remove an appointed estate trustee who is not performing their duties properly, is acting in bad faith, has a disqualifying conflict of interest, or whose conduct endangers the estate. The threshold for removal is whether the estate and its beneficiaries would be better served by replacing the trustee.

These applications are legal proceedings and can be expensive. Before committing to a court challenge, it is worth getting a clear assessment of the merits, what evidence you have, and whether there are practical alternatives such as negotiating a replacement trustee by agreement.