Theodore Grenville Ellis was born on September 25, 1829, in Boston, Massachusetts. He was a civil engineer in Hartford before the war.
On August 23, 1862 he became first lieutenant and adjutant of the 14th Connecticut Infantry Regiment. By July 1, 1863 he had advanced to major and was in command of the regiment at Gettysburg.
He eventually rose to colonel, then was brevetted brigadier general, U.S. Volunteers on March 13, 1865. He mustered out of volunteer service on June 23, 1865.
After the war he became surveyor-general of Connecticut.
He died on January 8, 1883, in Hartford, and is buried there in Cedar Hill Cemetery.