A runoff is required if no candidate wins more than 50 percent of the vote. Mr. Slocum drew 39 percent of the vote, and Ms. Masur placed second with 21 percent, according to semi-official results from the county Elections Office.
"You know what, my goal was to be in the top two and make it to November," she told the Almanac. "I'm very happy about it. Sixty percent of the county did not vote to support him. It's a new race. (The primary) race is over. It's going to be a lot of work and we're ready."
Asked for his take, Mr. Slocum paused, then suggested that his victory may have been due to name recognition and his "innovative" ways during 24 years as chief elections officer and assessor-county clerk-recorder. He retired in 2010. "Voters still remember some of my positive record," he said. "I crafted a message that represented (my priorities) and what I was all about and who I was. Maybe that message resonated with them."
Mr. Slocum and Ms. Masur, both residents of Redwood City, are running to replace Rose Jacobs Gibson in the District 4 seat on the Board of Supervisors. The district includes Menlo Park, East Palo Alto, Redwood City and the unincorporated communities of North Fair Oaks and Oak Knoll.
The election drew 85,227 votes, according to the latest count, with Mr. Slocum receiving 31,100 and Ms. Masur 18,126.
The rest were distributed among the five other candidates: Menlo Park Mayor Kirsten Keith, 12,774 (15 percent); East Palo Alto City Council member Carlos Romero, 7,565 (8.9 percent); county Board of Education member Memo Morantes, 6,830 (8 percent); Menlo Park City Councilman Andy Cohen, 4,152 (4.9 percent); and Redwood City Planning Commissioner Ernie Schmidt, 2,680 (3.1 percent).
Reelected and running unopposed were supervisors Dave Pine and Adrienne Tissier, who represent communities in the north part of the county. Supervisors must live in the district they represent, but they run for election county-wide.
Services the Board of Supervisors oversees include public health, the county jail, criminal prosecution, child support, care for the aged and people with disabilities and behavioral problems, protection of the environment and county parks, and housing for residents with financial hardships.
Supervisors receive an annual salary of $117,460. Mr. Slocum, who has a pension from his 24 years as chief elections officer and assessor-county clerk-recorder, said he would follow the example set by former longtime county sheriff and current supervisor, Don Horsley, and forego his salary.
Mr. Slocum, 64, is a 30-year resident of Redwood City. He and his wife Maria-Diaz-Slocum have two sons. He graduated from San Diego State University with a bachelor's degree in U. S. history, he told the Almanac.
Ms. Masur, 47, has a bachelor's degree from Macalester College and a master's degree in public health from Hunter College/City University of New York, she told the Almanac. She is the executive director of Teen Talk, a nonprofit organization devoted to preventing teen pregnancies, according to her online biography.
Among factors crucial to winning a county-wide election are fundraising, endorsements and name recognition.
Mr. Slocum likely won the name-recognition contest hands down. Anyone who registered to vote or had real property assessed between 1986 and 2010 in San Mateo County would have seen his name as the chief elections officer and assessor.
Ms. Masur beat Mr. Slocum soundly in convincing people to give her campaign money and endorse her candidacy.
She raised a total of $96,818, according to campaign finance reports. She had an early start, with $28,202 in contributions from the last half of 2011.
Mr. Slocum, who joined the race in early March, received $20,237 in campaign donations, but he is ready for a fight. His campaign's overall revenues include family loans of $102,683, finance reports show.
Ms. Masur, alone among the seven candidates, neither borrowed nor lent her campaign money, according to her reports.
She is also alone in the extent of her support from organized labor, including electrical workers, plumbers and steamfitters, firefighters and government employees.
Mr. Slocum's list of endorsements once included the San Mateo County Labor Council, but the council told him to remove it, he said in an interview. It was an "open" endorsement, meaning that the Labor Council allowed member unions to endorse him if they chose to. Apparently, none did.
Unions can help a candidate with funding for direct-mail campaigns and providing volunteers to go door-to-door, both important when running in a large metropolitan county.
If endorsements by current or former elected officials are valuable, Ms. Masur won that category, too. Her website lists 93, including four from Sacramento, 71 from San Mateo County, and 21 from elsewhere in the state.
Mr. Slocum's website listed endorsements from 13 elected officials, all from the county and exceeded by Menlo Park Mayor Kirsten Keith with 47 such endorsements and East Palo Alto City Councilman Carlos Romero with 22.