He was born in Boston, Massachusetts, the son of Nathaniel Langdon Frothingham (1793–1870), a prominent Unitarian preacher, and through his mother's family he was related to Phillips Brooks.
[5] Indeed, in 1864 he was recognized as leader of the radicals after his reply to Frederic Henry Hedge's address to the graduating students of the Divinity School on "Anti-Supernaturalism in the Pulpit.
Paralysis threatened him, and he never fully recovered his health.Frederic Henry Hedge In 1881, he returned to Boston, and devoted himself to literary work until his death there.
Always himself on the unpopular side and an able but thoroughly fair critic of the majority, he habitually underestimated his own worth; he was not only an anti-slavery leader when abolition was not popular even in New England, and a radical and rationalist when it was impossible for him to stay conveniently in the Unitarian Church, but he was the first president of the Free Religious Association (1867) and an early and ardent disciple of Darwin and Spencer.
He was a greater orator than a writer, and his sermons in New York were delivered to large audiences, averaging one thousand at the Masonic Temple, and were printed each week; in eloquence and in the charm of his spoken word he was probably surpassed in his day by none save George William Curtis.