Does a construction lien take priority over a mortgage registered on the property in Ontario?

It depends heavily on timing and the specific type of mortgage advance involved, and this is one of the more technical areas of Ontario construction lien law. As a general matter, a mortgage registered on title before any lienable work or materials were supplied to the improvement typically has priority over liens for that work up to the amount actually advanced before the lienholder's rights arose, while further advances made under that same mortgage after lien rights arise can be treated differently depending on whether the lender had notice of the liens and other Construction Act priority rules.

Because mortgage priority disputes often involve competing claims from multiple lienholders and a lender, all fighting over limited proceeds from a sale or the holdback fund, these disputes are genuinely fact- and document-intensive - the exact registration dates, advance dates, and what the lender knew and when all matter. If you're a lienholder concerned about a mortgage's priority over your claim, or a lender assessing exposure to liens on a project it's financing, this is an area where getting a lawyer to review the specific registration and advance history early is particularly important, rather than assuming a general rule of thumb applies to your situation.