Although a slave state whose sympathies were largely with the Confederacy, Maryland remained in the Union during the Civil War. There was a serious riot in Baltimore as Union troops passed through the city in April, 1861. A major engagement, the Battle of Antietam, was fought outside Sharpsburg in September, 1862.

Maryland came to be dominated in the latter half of the 19th century by railroad interests. A long struggle to free the state government from their influence led to notable reforms in the early 20th century. In 1930 Maryland and Virginia settled their dispute, known as the “oyster bed war,” over the boundary line along the lower Potomac and nearby waters. The Lane Memorial Bridge, connecting the western and eastern shores across Kent Island, was completed in 1952.

In the early 1960's, Maryland was the scene of serious racial disorders following civil rights demonstrations. In the 1970's, federal investigations into long-standing political corruption in the state led to the conviction of several officials, including Spiro T. Agnew, a former Maryland governor and then Vice President of the United States.

In the 1980's, a major program to combat pollution in Chesapeake Bay was undertaken. Antipollution efforts continued with legislation passed in 1998 to regulate run-off from industrial chicken farms after such run-off was linked to outbreaks of water-borne illnesses in several Chesapeake tributaries. The cleanup of Chesapeake Bay involves cooperation with several states and is expected to continue beyond 2010.

Maryland is now a national center for space research, development, and production, especially at the National Aeronautics and Space Administration's Goddard Space Flight Center in Greenbelt.