Don Horsley, who served as the sheriff of San Mateo County from 1993 to 2007, received the highest number of votes -- 23,980 -- in a race against four other candidates for the District 3 seat of Rich Gordon.
Gordon is seeking to fill the state Assembly seat of Ira Ruskin, who is being termed out of office.
Montara resident April Vargas garnered 15,069 votes and will face Horsley in a run-off election in November. A candidate must obtain at least 50 percent-plus-one of the votes in order to avoid a run-off election.
Sequoia Healthcare District board of directors member Jack Hickey garnered 10,105 votes, while San Carlos City Councilman Matt Grocott received 8,757 votes and San Carlos resident Michael Stogner received 4,243 votes.
Supervisor Carole Groom, who was appointed to the board mid-term in December 2008 after then-Supervisor Jerry Hill was elected to the state Assembly, appears to have defeated her sole opponent, retired businessman Daniel Kaul, with 42,505 votes in the District 2 race.
Kaul, who received 14,773 votes, said in the weeks prior to the election that he had stopped campaigning but would have served the seat if elected.