N.J. Stat. § 30:8-19
It shall be lawful for the board of chosen freeholders of any county in this state to assume and thereafter to exercise the custody, rule, keeping and charge of the county jails in their respective counties, and of the prisoners therein, whenever any such board shall decide, by the affirmative votes of two-thirds of all its members, so to do, and shall file a certificate of such decision attested by the director and the clerk of such board, in the office of the secretary of state. When such certificate shall have been so filed, the custody, rule, keeping and charge of the county jail in such county shall be no longer in the sheriff of that county, but in the board of chosen freeholders thereof, and in such jailer, keeper or warden as they shall appoint for that purpose.
N.J.S. § 30:8-19