[header] How to pay off someone else's mortgage [title] Familiarize yourself with gift tax law. [step] Any method of paying for someone else's mortgage would qualify as a gift. In the united states, if you give someone a certain amount of money without receiving a service in return, you become liable for the gift tax. 
The gift tax is the u.s. government's way of preventing people from avoiding income taxes by giving away all of their money.