Sidney is a city in and the county seat of Cheyenne County, Nebraska, United States. The city is nine miles north of the Colorado state line. …Sidney is a city in and the county seat of Cheyenne County, Nebraska, United States. The city is nine miles north of the Colorado state line. The population was 6,410 at the 2020 census. The city was named for Sidney Dillon, president of the Union Pacific Railroad. It was founded in 1867 by the Union Pacific and grew up around the military base of Fort Sidney, where soldiers were stationed to guard the transcontinental railroad against potential Native American attacks.