Coatesville is a city in Chester County, Pennsylvania, United States. The population was 13,100 at the 2010 census. Coatesville is approximately 39 miles west of Philadelphia. It developed along Lancaster Turnpike, beginning in the late 18th century. It spans Rt. 30, the "Main Line" highway that runs west of Philadelphia.Coatesville developed in the early 20th century with the growth of the Lukens Steel Company and other industry. Its population declined after industrial restructuring, which reduced these jobs. Lukens was bought by the Bethlehem Steel Corporation in 1997. In 2002, Bethlehem was bought by the then Ohio-based International Steel Group. Later, Mittal Steel bought ISG and merged with Arcelor Steel to form the ArcelorMittal company.