- He was a lawyer. During the War, however, he was a trusted staff officer, military secretary, and aide for General Lee from March of 1862 until the surrender at Appomattox. He wrote his memoirs as An aide-de-camp of Robert E. Lee, published in 1927 (ed. Sir Frederick Maurice). After the war Marshall was a key figure in summoning the ghost of Lee's presence for the Lost Cause. He worked on a biography of Lee.
He was at odds with aspects of the Lost Cause arguments, however. In 1896, he gave an address on "the Events leading up to the Battle of Gettysburg" which was published in the Richmond Dispatch in January of 1896; he made here a key argument for J.E.B. Stuart's complicity in leaving Lee without information on Union troop movements before the battle. A rebuttal was published by Col. John Mosby, of the Virginia cavalry, who (along with the arguments by Jubal Early) put the onus for the loss on Longstreet--which was, historians have argued, politically motivated, since after the war Longstreet joined the Republican party and worked for the new goverment in Washington.
He was a relative of General George Marshall; his great uncle was Chief Justice John Marshall.
For one book which has mention of his relationship with Lee after the war, see Lee, the Last Years, by Charles Bracelen Flood, which is a biography of Lee from Appomattox to his death in 1870.