Mert County Police Department, founded in 1776, located approximately just over 18 miles away from Hershel Greene's Farm is a rural law enforcement similar to the King County Sheriff's Department and Linden County Sheriff's Department. They operate in Mert County using light grey Ford Crown Victoria Police Interceptors and police bikes to patrol the county highways and streets. They're only seen in the episode "18 Miles Out" of AMC's The Walking Dead.
Not much is known about the Mert County Sheriff's Department except that they were in charge of law enforcement in Mert County, Georgia.
After the outbreak, officers most likely tried to assist FEMA and the U.S. Military in evacuating civilians, however, Mert County police cruisers and bikes are seen at an unknown public works station.