Old Catonsville High School, also known as St. Mark's School, is a historic school building located at Catonsville, Baltimore County, Maryland.

It is a masonry schoolhouse which consists of a one-story, single bay structure (built in 1878) and a three-story, four bay addition (built in 1898).

In 1885 the school was designated a high school and in 1910 the property was sold to the Catholic Church.

[2] It was listed on the National Register of Historic Places in 1987.

This article about a Registered Historic Place in Baltimore County, Maryland is a stub.