Ericsson is chiefly remembered as the designer and builder of the Monitor, a radical departure from previous types; and its fortuitous conflict with the Virginia during the Civil War, less than five months after its keel was laid, caught the imagination of the people and made Ericsson a hero (see Monitor and Merrimack. With his associates he was busy the remainder of the war designing and building other ironclad vessels, and after the war he built monitors for other governments until the type was abandoned. He also constructed gunboats for Spain, and worked on a "destroyer" with successful devices for releasing torpedoes underwater, but he could not interest the U.S. government in it.

Ericsson made many other contributions to engineering, notably in ordnance, in marine engines, and in caloric or heat engines. In his late years he did experimental work in solar physics.