In the heat of battle, Major Winthrop leapt onto the trunk of a fallen tree and reportedly yelled, "One more charge boys, and the day is ours." Soon thereafter, he was killed by a musket ball to the heart and became the first casualty of rank for the Northern side in what history regards as the first pitched land battle of the Civil War. Ironically, ardent abolitionist Winthrop may have been shot by the African-American slave of a Confederate officer in the 1st North Carolina Infantry. (Three different soldiers, as well as this slave, referred to in the records only as "Sam," claimed to have killed him.)
Winthrop's novels, for which he had failed to find a publisher during his lifetime, appeared posthumously. They include John Brent, founded on his experiences in the far West, and Edwin Brothertoft, a story of the American Revolution. Cecil Dreeme, his most important work, was a semi-autobiographical novel dealing with social mores and gender roles set at New York University, where Winthrop had once been a lodger. Other works include The Canoe and the Saddle and Life in the Open Air. His sister, Laura Winthrop Johnson, assembled a collection of his poems and prose organized by the time period of his life Life and Poems of Theodore Winthrop.