John Ericsson developed a career in mechanical engineering that linked Swedish surveying, British invention, and American naval construction. After early technical work and service in the Swedish army, he moved to England in 1826. He designed engines, a steam fire engine, and the locomotive Novelty, which competed at the Rainhill trials in 1829. His experiments with marine screw propulsion offered an alternative to exposed paddle wheels.

Ericsson moved to the United States in 1839 and worked with naval officer Robert F. Stockton on the steam warship Princeton. Its machinery and propulsion arrangements demonstrated important possibilities for warship design. In 1844 an experimental gun exploded aboard the ship, killing senior officials and others. The weapon that burst was not Ericsson's design, but the disaster and his dispute with Stockton damaged his position and complicated payment for his work.

During the Civil War, Ericsson designed the Union ironclad Monitor, combining a low hull, steam machinery, and a revolving armored gun turret. Built with industrial and financial partners, it entered service in 1862 and fought the Confederate Virginia at Hampton Roads on March 9. The battle was indecisive tactically but helped demonstrate how armored steam vessels could transform naval warfare. The design's advantages were accompanied by serious limitations in seaworthiness; Monitor sank later that year in heavy weather.

Ericsson continued developing monitor-type warships, engines, and other mechanical systems. His interests included hot-air engines and solar energy, reflecting interest in converting heat into work. Some projects proved impractical commercially even when technically inventive. He died in New York in 1889, and his remains were returned to Sweden. His achievements depended on experimentation, patents, skilled manufacturing, and collaboration as well as individual design. Accidents, credit disputes, and wartime demands repeatedly reshaped his reputation.