The executive director of a community mental health services program shall function as the chief executive and administrative officer of the program and shall execute and administer the program in accordance with the approved annual plan and operating budget, the general policy guidelines established by the board, the applicable governmental procedures and policies, and the provisions of this act. The executive director has the authority and responsibility for supervising all employees. The terms and conditions of an executive director’s employment, including tenure of service, shall be as mutually agreed to by the board and the executive director and shall be specified in a written contract.