Present day Catonsville was settled in the 18th century. In the early 19th century, a county road along the Patapsco River—named the Frederick Turnpike, later designated Route 144—was opened by the Ellicott family to service traffic between their flour mill, Ellicott Mills, and Baltimore. Catonsville as we know it today was settled along this route by Richard Caton, under the authority of his father-in-law Charles Carroll, a signer of the Declaration of Independence. Travelers along "the turnpike" (as it was then known) rested and conducted business in the area, causing Catonsville to grow.
The large Victorian and Colonial homes located in Catonsville were built by wealthy Baltimoreans. Originally, these communities were used as summer residences to escape the heat in Baltimore. Eventually, as in many communities with the introduction of the automobile and electric trolley, families began to reside in Catonsville year round. Baltimore has attempted over the years to annex Catonsville, the last attempt in 1918, but all attempts were rebuffed. The community remains an unincorporated town in Baltimore County. It is home to Spring Grove Hospital Center, the nation's second oldest continuously operating psychiatric hospital, as well as the University of Maryland, Baltimore County.
Samuel Clemens (Mark Twain) made his last public appearance on June 9, 1909, at the commencement ceremony of St. Timothy's School for Girls in Catonsville, to fulfill a promise he made to a young girl he had met on the steamer "Minnehaha" in 1907.
Catonsville was briefly made quite famous during the 1968 protest by the "Catonsville Nine", during which draft records were burned by Catholic anti-war activists.
In 2002, the Maryland legislature issued a proclamation declaring Catonsville to be "Music City, Maryland" due to a concentration of musical retail stores, venues and educational facilities in the area.
In 2007 Money magazine ranked Catonsville the 49th best place to live in the USA, third best in Maryland and Virginia.
Catonsville is a terminus of the Trolley Line Number 9 Trail.