Cal. Gov. Code § 65063.7

No supervisor, mayor, or city council member shall hold office on the regional planning board after ceasing to hold the office of supervisor, mayor, or city council member, respectively, and that person’s membership on the board shall thereafter be considered vacant, except that any mayor who continues to hold office as a city council member, or city council member who continues to hold office as mayor, shall not be considered to have ceased to hold office under this section.