Mortgage company holding land as collateral that's not legally owned by me, is their contract void?

Assets used to secure a loan do not necessarily have to belong to the borrower.  Family members cosign loans, or pledge collateral to secure loans, all the time.  If what you're saying, however, is that you told the morgagee that you owned the land, and used it as security, without the real owner's knowledge or authority to do so, than the mortgage lien might be invalid (it's not certain - the mortgagee may claim that you had apparent authority to pledge the land, or the real owner may have ratified the arrangement, or be estopped to challenge the transaction, because he/she knew about it but never said anything to the mortgagee. or many other arguments depending on the full facts), but you yourself would still be liable on the debt AND may face criminal charges for fraud and forgery.