The chief law-enforcement officer in county governments is the

Sheriff

Explanation:

A sheriff is the chief law-enforcement officer in a county in the United States. He is responsible for keeping order in his county and to guarantee its citizens are protected and safe. A sheriff is usually chosen in an election, he is accountable directly to the state's constitution and  ultimately to the United States Constitution.

In some states and counties, sheriffs are also chiefs of police and they manage the county's jails. A sheriff has a deputy who is a regular officer in the sheriff's office.

Of the 50 states in the USA, 48 have sheriffs, with the exception of Alaska and Connecticut.