Ernston is a neighborhood in Sayreville in Middlesex County, New Jersey, United States. It was a stop on the Camden and Amboy Railroad.In 1876, when the newly-formed Township of Sayreville was created from approximately 14 square miles (36 km2) of South Amboy's surroundings, Ernston was consolidated with Morgan, Melrose, and Sayre's Village under one municipal government.