sidneySidney is a city in Shelby County, Ohio, United States, and its county seat. [4] The population was 20,421 at the 2020 census.It is approximately 36 miles (58 km) north of Dayton and 100 milesSidney is a town in Delaware County, New York, United States. The population was 5,536 at the 2020 census. [3] [4] The town is at the northwestern corner of the county and contains