Sheriff is not an elected position in Fairfax County. It’s an appointment by the Democrat-controlled Board of Supervisors. He/she is responsible for the jails and the courts. The Chief of Police is a separate post, also appointed.
The Fairfax County Sheriff is an elected position. The Fairfax County Police Chief is not. They are two separate entities and the Sheriff's Office take precedent over the county police.
From their web site: "Sheriff Stacey Ann Kincaid is the first woman to lead the Fairfax County Sheriff’s Office in its 279-year history. She won a special election in 2013 to fill the balance of a term and was re-elected in 2015 and 2019 to full four-year terms. She serves over 1.1 million residents in Fairfax County, the City of Fairfax and the Towns of Herndon and Vienna."