Shirlee Zane, who won the 3rd District seat held by Tim Smith, who retired, was administered the oath of office by Assemblywoman Noreen Evans, D-Santa Rosa.
Zane told the standing-room-only crowd that government works best when it brings people together and that it must never be arrogant, self-serving or wasteful.
Former state Sen. Art Torres administered the oath of office to Efren Carrillo, 27, who won the 5th District seat held by Mike Reilly, who did not seek re-election.
Carrillo said he grew up in the first Habitat for Humanity house in Sonoma County. He is the first Latino elected to the five-member board.
"A small investment in youth brings a large return," Carrillo said. "I'm looking forward to tapping the wisdom of our youth."
First District Supervisor Valerie Brown, who was re-elected in November, also was sworn in Tuesday morning. She said county government "is where the rubber meets the road" and that satisfaction comes when "at the end of the day we feel we've accomplished something."
Supervisor Paul Kelley was chosen chairman of the board and Brown was selected as vice chair. The fifth member of the board is Mike Kearns.
