Upon graduating from Harvard College and Divinity School, this son of the minister of the First Church in Boston was ordained at the North Church in Salem in 1847. His decision to resign in 1855 was triggered by the reenslavement of Anthony Burns under the Fugitive Slave Act. Attacking Christianity for its complicity with slavery, he refused to administer communion.

When minister of the Independent Liberal Church of New York in 1859, he gathered a large congregation of hearers. When a little group of radicals formed the Free Religious Association, they persuaded him to be president to lend visibility and weight to their undertaking.

Frothingham’s continuing career as a biographer and historian brought forth major works such as: The Religion of Humanity, Theodore Parker, Transcendentalism in New England, and Boston Unitarianism: 1820-1850.

Upon returning to Boston after retiring, he attended the First Church in Boston (Unitarian) which his father had served.