Napa County supervisors on Tuesday accepted Sheriff John Robertsonâs resignation effective June 26 and tentatively appointed Capt. Oscar Ortiz, chief of the American Canyon Police Department, to finish out his term. Robertson didnât go to the microphone during the session at Tuesdayâs Board of Supervisors meeting. Asked about his pending retirement before the session, he said he wanted to wait until the Board took action on his successor before making a statement. Support local news coverage and the people who report it by subscribing to the Napa Valley Register. Special offer: $5 for your first 5 months! Robertson began his law enforcement career in 1981 with the San Mateo Police Department. He joined the Napa County Sheriffâs Office in 1991 and was appointed sheriff by the Board of Supervisors in 2012 and elected in 2014 and 2018.