In 1847 Beecher was hired as a minister by Plymouth Church, recently founded by liberal Congregationalists in Brooklyn, New York. Under his tutelage the congregation grew large, and his sermons helped ignite a religious revival in 1857. He gained an even wider audience by writing columns for the Congregationalist newspaper, The Independent, in which he expressed an uplifting, liberal, Romantic version of Christianity that emphasized God’s love and downplayed sin. Those columns were collected and republished in three volumes: Star Papers; or, Experiences of Art and Nature (1855), Plain and Pleasant Talk about Fruits, Flowers and Farming (1859), and Eyes and Ears (1862).

Beecher attracted further notoriety during the 1850s for his participation in the antislavery movement. He condemned the tougher Fugitive Slave Act, which was part of the Compromise of 1850, and the Kansas-Nebraska Act of 1854, which opened the Western territories to slavery. Guns used by the antislavery forces in Kansas were sarcastically referred to as "Beecher’s Bibles." He campaigned for the new antislavery Republican party and soon became influential in its ranks. When the Civil War broke out he wholeheartedly supported the Union cause, joined other abolitionists in pushing President Lincoln toward enacting emancipation, and traveled on a speaking tour of England to help stem that nation’s aid to the Confederacy. His patriotic speeches, which equated the Union cause with God’s will, were collected and published in two volumes, Freedom and War (1863) and Patriotic Addresses (1887).

In 1867 Beecher published Norwood, a novel of moral uplift set in the Civil War era which revolves around the lives of two sets of young lovers, one Northern and one Southern. It touted the author’s familiar motifs of God’s abundant love, the Romantic notion of divine immanence in nature, and the necessity of morality in everyday life. Beecher reached the pinnacle of his career in the 1870s. He began editing a new periodical, The Christian Union (later called The Outlook), which expressed his liberal, evangelical, Protestant perspective, and he published The Life of Jesus Christ (1871), Yale Lectures on Preaching (3 vols., 1872-1874), and his collected sermons, Plymouth Pulpit (19 vols., 1868-1884). He earned a handsome salary by traveling widely as a public speaker (e.g., 27,000 miles through 18 states and Canada in 1875, earning $60,000).

In the fall of 1872 Beecher’s stature and popularity as a religious and civic leader were threatened when Victoria Woodhull publicly charged that he had been having an affair with Elizabeth Tilton, the wife of Theodore Tilton, editor of The Independent. Woodhull, a leader of the free-love movement, was angered by Beecher’s hypocrisy in refusing to endorse free-love while secretly practicing it. She was also a friend of Theodore Tilton, who had written a favorable biography of her in 1871. The titillating story of an extramarital affair between the country’s foremost cleric and the wife of a well-known editor created a national sensation. Beecher denied the allegation and a board of inquiry at Plymouth Church exonerated him of any moral wrongdoing. (Mrs. Tilton at first confessed, then retracted her confession, then later confessed again.) Press scrutiny and public interest, however, escalated when Tilton sued Beecher. The trial commenced in January 1875 and ended with a hung jury six months later. The next year a second committee at Plymouth Church again refused to condemn their minister.