Can an unmarried mother change a baby's last name without the father's consent?

She should visit the local probate and family court to ask about the requirements in her jurisdiction.

If the father has established his paternity legally then she must obtain his consent. She will need to answer questions about the child's father when she requests the name change and will need to attest that her answers are true. The court may require that the father be given notice in case he has any objections.