Millbrook Township is a civil township of Mecosta County in the U.S. state of Michigan. The population was 1,064 at the 2020 census. The township is in the southeast corner of Mecosta County, bordered to the east by Isabella County and to the south by Montcalm. The unincorporated community of Millbrook is in the northeast corner of the township. Sta…Millbrook Township is a civil township of Mecosta County in the U.S. state of Michigan. The population was 1,064 at the 2020 census. The township is in the southeast corner of Mecosta County, bordered to the east by Isabella County and to the south by Montcalm. The unincorporated community of Millbrook is in the northeast corner of the township. State highway M-66 crosses the center of the township from north to south. Big Rapids, the Mecosta county seat, is 30 miles by road to the northwest.