A first mortgage is a primary lien on a piece of real estate. In the case of default, the loan has priority over all other liens or claims on the property because it is the principal loan that pays for the property. A first mortgage is not a borrower’s first home; instead, it is the first mortgage taken out on any single property. It is also known as a First Lien. If the house is refinanced, the refinanced mortgage becomes the initial mortgage.