Cal. Gov't Code § 1099

(a) A public officer, including, but not limited to, an appointed or elected member of a governmental board, commission, committee, or other body, shall not simultaneously hold two public offices that are incompatible. Offices are incompatible when any of the following circumstances are present, unless simultaneous holding of the particular offices is compelled or expressly authorized by law:
1. (1) Either of the offices may audit, overrule, remove members of, dismiss employees of, or exercise supervisory powers over the other office or body.
2. (2) Based on the powers and jurisdiction of the offices, there is a possibility of a significant clash of duties or loyalties between the offices.
3. (3) Public policy considerations make it improper for one person to hold both offices.
(b) When two public offices are incompatible, a public officer shall be deemed to have forfeited the first office upon acceding to the second. This provision is enforceable pursuant to Section 803 of the Code of Civil Procedure.
(c) This section does not apply to a position of employment, including a civil service position.
(d) This section shall not apply to a governmental body that has only advisory powers.
(e) For purposes of paragraph (1) of subdivision (a), a member of a multimember body holds an office that may audit, overrule, remove members of, dismiss employees of, or exercise supervisory powers over another office when the body has any of these powers over the other office or over a multimember body that includes that other office.
(f) This section codifies the common law rule prohibiting an individual from holding incompatible public offices.