His death was announced Saturday by the County Executive's Office.
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Horsley served as sheriff from 1993 to 2007 and was elected a supervisor in 2010. He was re-elected twice and retired in 2022.
Horsley was born in San Francisco. His father, a laborer in the city's produce markets, suffered a stroke when Horsley was 12.
His mother was a bookkeeper, and his grandmother was confined to a mental hospital, the county executive's office said in a statement.
"It was all concrete and slamming doors," Horsley said in April 2021 at the groundbreaking for a new mental health center, according to the executive's office .
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"Because of that, being a little boy, seeing a state hospital and women who are essentially locked up in a concrete monstrosity, I guess touched something inside me such that I was always interested in doing better for people who have mental health issues."
As a youth, Horsley moved with his family to Daly City, where he attended Westmoor High School. He received a bachelor's degree from San Francisco State University.
He was a middle school teacher and a juvenile counselor for the San Mateo County Probation Department.
Horsley served as a police officer in Daly City and Pacifica before joining the San Mateo County Sheriff's Office in 1972.
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Information about services and survivors was not immediately available, according to the county executive's office.
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