Mont. Code § 76-1-223
When a city council has enacted an ordinance creating a city planning board or when a vacancy occurs in the county's membership on the city planning board, the board of county commissioners of the county in which the city is located shall within 45 days designate a representative of the county to the mayor of the city for appointment to the city planning board. This representative may be a member of the board of county commissioners or an officeholder or employee of the county. The mayor may not reject or refuse to appoint to the city planning board a representative designated by a board of county commissioners as provided in this section, but if the county fails to designate a representative, then the mayor may appoint as a representative of the county a person of the mayor's own choosing and at the mayor's sole discretion.
§ 76-1-223, MCA