Marshall served as an aide de camp, assistant adjutant general and military secretary to Gen. Robert E. Lee, and later worked to establish the Lost Cause and monuments to his former comrades.

His sister Catherine Taylor Marshall (1832-1866) would marry Fendall Dade Marbury of Prince George's County, Maryland.

He received a private education at the Richard M. Smith School and the Warren Green Academy (both in Warrenton) before entering the University of Virginia in Charlottesville.

In 1860, he and his wife were living with his Irish-born father-in-law and architect brother-in-law and his family in a large household which employed six white domestic servants but no slaves.

He also located the Wilmer McLean house where the surrender meeting took place and drafted Lee's "Farewell Order" to the Army of Northern Virginia.

Marshall delivered the dedication addresses for the monuments to Lee in Richmond, Virginia, and Ulysses S. Grant in New York City.