N.Y. Second Class Cities Law § 12
There shall be appointed by the mayor a corporation counsel, city engineer, commissioner of public works, commissioner of public safety, commissioner of public welfare and sealer of weights and measures. The commissioner of public welfare shall be appointed in accordance with the provisions of section one hundred sixteen of the social welfare law. Other officers may be appointed as provided in this chapter or otherwise by law. All appointments to any city office shall be evidenced by a certificate in writing, signed by the appointing officer and filed forthwith in the office of the city clerk. If an appointment be made by the common council such certificate shall be signed by the officer presiding at the time the appointment was made and attested by the city clerk.
N.Y. Second Class Cities Law § 12