Im a permanent resident and my mother became a citizen before i turned 18. Would derivative citizenship apply in my case? How would i claim?

If you were living together with your mother with permanent residence and in her legal custody when your mother became a citizen, you would have become a US citizen at the time that she became one. You would claim it by either filing for a US passport application or filing for an N-600 application for certificate of citizenship. 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.