[1] After his apprenticeship Leavitt worked for one year at the engineering firm of Corliss & Nightingale in Providence, Rhode Island before returning to Boston, where he became assistant foreman for Harrison Loring.

From 1859–61 he again worked in Providence, this time as chief draftsman for Thurston, Gardner & Company, builders of steam engines.

During the Civil War, Leavitt first served aboard the USS Sagamore, then in construction roles in Baltimore, Boston, and Brooklyn, and ultimately as an instructor in steam engineering at the United States Naval Academy.

In 1884 he received the first honorary doctorate of engineering degree granted by the Stevens Institute of Technology.

For the company's smelting plant on the Upper Peninsula of Michigan, Leavitt also designed devices known as sand wheels, which lifted copper tailings and sent them sliding down a sluiceway into nearby Torch Lake.