N.H. Rev. Stat. Ann. § 48:18

In any city which adopts the provisions of this subdivision as provided in RSA 48:20, the mayor, with the approval of the board of aldermen in cities with a strong mayor form of government, or the city manager, with approval of the city council in cities with a weak mayor form of government, shall appoint a director of human services who shall hold office for a term of 4 years and until a successor is appointed and qualified. A vacancy shall be filled for the unexpired term.