Through his father, he was descended from Governor John Winthrop, and through his mother, from theologian Jonathan Edwards, as well as early settlers George (Joris) Woolsey and Thomas Cornell.

[2] After contributing to periodicals, short sketches, and stories, which attracted little attention, Winthrop enlisted in the 7th Regiment, New York State Militia, an early volunteer unit of the Federal Army that answered President Abraham Lincoln's call for troops in 1861.

After a Federal attack to the enemy right flank was foiled, Winthrop led an ill-fated assault on the Confederate left held by four companies of the 1st Regiment North Carolina Infantry, under the command of Colonel (later Lieutenant General) Daniel Harvey Hill.

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.

Works published during Winthrop's lifetime include a pamphlet accompanying the painting The Heart of the Andes by his friend Frederic Edwin Church, distributed in 1859.