Cushing began his military career as a sergeant in the 6th Continental Regiment in January 1776. He was commissioned a 2nd lieutenant in the 1st Massachusetts Regiment in January 1777 and was promoted to 1st Lieuteant in January 1778. He was taken prisoner in May 1781 and was later exchanged. He was breveted to the rank of Captain in September 1783. Cushing was one of the last soldiers to be dischaged from the Continental Army in June 1784.
On March 4, 1791 Cushing was commissioned a Captain in the 2nd Infantry Regiment. He was promoted to Lieutenant Colonel of the 2nd Infantry on April 1, 1802. He was promoted to Colonel of the same regiment on September 7, 1805.
Cushing served as Adjutant General of the U.S. Army from 1797 to 1798 (acting), 1800 to 1807, and 1812 to 1813. He also served as acting Inspector General of the U.S. Army from 1800 to 1807 and 1812 to 1813. Cushing was promoted to the rank of Brigadier General in 1812 and retired in June 1815.
In January 1816 Cushing was appointed collector of customs for the port of New London, Connecticut, succeeding Jedidiah Huntington. In 1817, Cushing fought a duel with Virginia congressman William J. Lewis and was saved when the bullet struck his watch. The two resolved their differences, and Lewis, stepping up to the general, said: “I congratulate you, general, on having a watch that will keep time from eternity.”
Cushing was an original member of the Society of the Cincinnati from Massachusetts and was a member of the Society from 1783 until his death in New London, Connecticut in 1822. In 1799, he commissioned artist James Peale to create a miniature portrait of himself.