Sidney is a city in Fremont County, Iowa, United States. The population was 1,070 at the time of the 2020 census. It is the county seat of Freā¦Sidney is a city in Fremont County, Iowa, United States. The population was 1,070 at the time of the 2020 census. It is the county seat of Fremont County and is one of the smallest county seats in the state.