Unadilla was formed by an act of the Legislative Council of the Territory of Michigan on March 26, 1835. The township was at first attached to Washtenaw County. On March 6, 1838, Iosco Township was set off and with the southern portion being assigned to Lyndon Township in Washtenaw County. At this time, Unadilla assumed its present boundaries. Unadilla was one of the orginal townships of Livingston County. It lies in the southwest corner of the county, bounded on the north by Iosco Township, on the east by Putnam Township, on the south by Lyndon Township, Washtenaw County, and Waterloo Township, Jackson County and on the west by Stockbridge Township, Ingham County. By survey it is township 1 north, range 3 east. Before 1880 the township contained three villages, Unadilla, Williamsville, and Plainfield located respectively in sections 35, 28, and 5. In 1884 the village of Gregory began as a station on the Grand Trunk Western Railroad. Today the Township Hall is located in Gregory.
The land in the township is greatly diversified. There are considerable hills along with extensive marshes. A large portion of the northwest portion is a beautiful plain. On this plain is located the village of Plainfield. The Portage River and its tributaries run through this part of Unadilla Township. The township continues to be a farming community.