Ilchester is an unincorporated community and census-designated place in Howard County, Maryland, United States. It was named after the village of Ilchester in the English county of Somerset. Ilchester is situated in Howard County, Maryland, between Ellicott City to the northwest and Elkridge to the southeast. Columbia is to the southwest. The Patapsco River forms the northeast boundary of the community and serves as the county line, with Baltimore County and the community of Catonsville across the river.
The original mill community of Ilchester is located along the Patapsco River at the junction of Ilchester Road and Bonnie Branch Road. However, the current CDP extends southwest to include the suburban terrain of Howard County outside of the Patapsco River valley. Maryland Route 103, Montgomery Road, is the main local road through the CDP. Maryland Route 100 is a freeway that runs through the southern part of Ilchester, serving the community at Exit 4 with Montgomery Road. To the south, Route 100 intersects with Interstate 95, which forms the southeastern boundary of the Ilchester CDP.
The season four finale of the television show Supernatural took place at a fictional “St Mary’s Convent” in Ilchester. This location is based on the real St. Mary’s College, which stood overlooking Ilchester Road near the B&O railroad and Patapsco River.