 Immigrating to the United States, he fought for the Confederacy during the American Civil War, and was a leader of a notorious plot to seize control of parts of the Northern U.S.

Grenfell was born in London, England.He came to America in 1862 and became an officer in the Confederate States Army, serving with cavalryman John Hunt Morgan, General Braxton Bragg, and General J.E.B.Stuart.