He served twenty months in Captain Theodore Bliss's company of the 26th Massachusetts Infantry Regiment, playing the fife in the Continental Army from 1775 to 1778.

A letter from Greenwood to Lt. General George Washington on his denture charges, dated 1799, is in the A.D. Black History of Dentistry Collection at Northwestern University.

In May, Greenwood heard the news and alone walked 150 miles (240 km) from Falmouth to Boston, occasionally stopping at taverns to play music for soldiers.

He witnessed struggling wounded men returning to Cambridge on foot, which initially discouraged him from continuing service.

He was encouraged to continue, however, after apparently witnessing a wounded black soldier, who planned to return to battle after fixing his injuries.