Cal. Gov. Code § 51501

By ordinance, a city legislative body may transfer the duties of the city treasurer to the treasurer of the county in which the city is situated, and shall, by January 1, 1969, transfer the assessment and tax collection duties performed by the city assessor and tax collector to the assessor and tax collector of the county in which the city is situated, and no city shall establish the offices of city assessor and tax collector after that date.