Auburn is a city in Bay County, Michigan, United States. The city's population was 2,068 at the 2020 Census. It is included in the Sagina…Auburn is a city in Bay County, Michigan, United States. The city's population was 2,068 at the 2020 Census. It is included in the Saginaw, Midland, and Bay City metropolitan area. It was first settled in 1854. On February 26, 1869, the settlement was granted a post office with the name of Skinner. On November 19, 1877, the Skinner Post Office was renamed Auburn. The unincorporated community incorporated as a city in 1947. The default Zip Code of 48611 is shared by Williams Township, and much of southwestern Bay County.