His death was announced Saturday by the County Executive's Office.
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.
Information about services and survivors was not immediately available, according to the county executive's office.
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