Legal Question in Real Estate Law in California

The correct answer here may depend upon what you mean by "mortgage documents" and "on behalf of," and whether you had knowledge of your father's death at the time you signed the documents. Also, does or did your authority to sign on his behalf arise because you were his agent, or did you have a power of attorney? An agent or a person acting under a power of attorney can, in some circumstances, sign after the principal's death if he/she has no knowledge of the death (see Civil Code section 2356 re agents and Probate Code sections 4152(a)(4) and 4304 re attorneys in fact). In general, however, I'd say that signing documents on behalf of a person you know to be deceased is unwise as possibly fraudulent, especially if the other party is unaware of the situation. As a general matter, it's the right of an executor or administrator, duly appointed by the probate court, to handle the business affairs of a deceased person. As noted above, there can be exceptions.

Mr. Whipple gives an interesting look into obscure and arcane legal rules. I'll give you a simple answer. If you know your father is dead and you sign his name or on his behalf, at a minimum what you sign is void, it may be fraud, and it may even be a crime. In now way shape or form is it legal.