Are there special tax filing complications if the executor of an Ontario estate is a US citizen?

Potentially, yes - being a US citizen can carry US tax filing obligations that follow the person regardless of where they live or what role they're serving in, including acting as an estate trustee for an Ontario estate. This is a feature of US tax law generally, not something created by Ontario or Canadian estate rules, and it applies to the individual's own status rather than to the Ontario estate itself simply because of who was appointed trustee.

Whether this creates any actual complication for the Ontario estate's administration, as opposed to just the individual trustee's personal US filings, depends on the specific facts. This content isn't the place to summarize US tax thresholds, forms, or reporting rules, since that's foreign law outside Ontario and Canadian scope and details here could easily go stale or be wrong. What matters practically is recognizing early that a US-citizen trustee's personal tax situation is a separate question from the estate's Canadian tax obligations, and that both may need attention.

If you're naming a US citizen as your estate trustee, or you are one acting for an Ontario estate, raising the US angle with a cross-border tax advisor, alongside the estate's regular Canadian accountant or lawyer, is worth doing early rather than assuming it's a non-issue.