Farrell is a city in western Mercer County, Pennsylvania, United States, along the Shenango River. The population was 4,258 at the 2020 ce…Farrell is a city in western Mercer County, Pennsylvania, United States, along the Shenango River. The population was 4,258 at the 2020 census. It is part of the Hermitage micropolitan area. Once dubbed "The Magic City," Farrell sprang up practically overnight when a steel mill was constructed in 1901 on a plain bordering the Shenango River, near Sharon, in what was then part of Hickory Township.