Not all legal documents need to be witnessed, but if you have a legal document, such as a mortgage or other type of contract, a witnessing signature will provide evidentiary support in case there's a dispute about who signed. Generally, a witness must be a disinterested third party, including an attorney or a notary public.
TL;DR (Too Long; Didn't Read)
Any disinterested third party may be a witness to a legal document, including a notary public or an attorney, as long as the witness is 18 years old or older.
The Purpose of a Witness
Individuals who serve as witnesses to legal documents verify that the signature on the document belongs to the person with that name. In other words, witnesses protect against forgery. If a question regarding the signature on a legal document comes up, a witness may be called upon to testify in court that the individual whose name is on the legal document signed the document in their presence.
Legal Documents that May Need Witnesses
Depending on where you live, the court may request or require that certain legal documents be witnessed by one or more people. Legal documents that often need witnesses include wills, mortgages, contracts, divorce decrees and other property settlement documents.
Who Can Be a Witness
A lawyer, a notary public or a third-party without an interest in the document may serve as a witness to a legal document. In some states, a lawyer's or notary's signature may be required on certain documents to limit the chance of forgery. As long as the person is disinterested (that is, has no interest in the outcome of any process or proceeding related to the document), at least 18 years old and of sound mind, she can be a witness.
Who Shouldn't Be a Witness
Your spouse or another member of your family should not serve as a witness to any legal document you sign. Even if neither party is named in the document, your spouse and any relatives still have an interest in your property or will have some interest in the outcome of a lawsuit if one occurs. Family members by marriage are also perceived to be interested parties and shouldn't be witnesses to legal documents. The key is to find a person who is impartial.
Other Witness Requirements
Any individual named in a legal document cannot act as a witness to that document. So, if you named your best friend in your will, he cannot serve as a witness. This is simply because he's an interested party, and an interested party cannot be a witness, because they cannot be impartial. Also, all witnesses to legal documents must be over the age of 18 at the time they witness your signature, and they must be of sound mind.