Mike McAuliffe was sworn in Thursday morning.
He is running unopposed in next week's election, so county commissioners asked him to take over the position a couple of months early.
Former JP Bryan Smith had his last day in office on Wednesday.
Judge McAullife will also be sworn in on January 1 to fulfill his elected four year term.
McAuliffe served as Abilene High School's resource officer for more than 20 years.