Did I automatically become a US citizen when my mother became a US citizen prior to my 18th birthday?

Unfortunately you did not automatically become a U.S. citizen when your mother became a U.S. citizen prior to your 18th birthday in June 1996. Your mother would have had to take affirmative steps and file Form N-600 application for certificate of citizenship and you would have had to become a citizen before the age of 18. I note that the Child Citizenship Act did not become effective until February 27, 2001. If you were under the age of 18 on that date, you would have automatically become a U.S. citizen if you were a permanent resident and in the legal and physical custody of a U.S. citizen parent. The last provision applied even where the child's parents were living together and the other was not a U.S. citizen. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.