Born in Detroit, Michigan, he was the son of David Bacon (1771-1817), a missionary among the Indians in Michigan, and founder of the town of Tallmadge, Ohio. The son prepared for college at the Hartford, Connecticut grammar school, graduated from Yale University in 1820, and from the Andover Theological Seminary in 1823. From 1825 until his death he was pastor of the First Church (Congregational) in New Haven, Connecticut, occupying a pulpit which was one of the most conspicuous in New England, and which had been rendered famous by his predecessors, Moses Stuart and Nathaniel W. Taylor. In 1866, however, though never dismissed by a council from his connection with that church, he gave up the active pastorate.
From 1826 to 1838, he was an editor of the Christian Spectator (New Haven); was one of the founders (1843) of the New Englander (later the Yale Review); founded in 1848 with Dr R. S. Storrs, Joshua Leavitt, Dr Joseph P. Thompson and Henry C. Bowen, primarily to combat slavery extension, the Independent, of which he was an editor until 1863; and was acting professor of didactic theology in the theological department of Yale University from 1866 to 1871, and lecturer on church polity and American church history from 1871 until his death.
Gradually, after taking up his pastorate, he gained greater and greater influence in his denomination, until he came to be regarded as perhaps the most prominent Congregationalist of his time, and was sometimes popularly referred to as "The Congregational Pope of New England." In all the heated theological controversies of the day, particularly the long and bitter one concerning the views put forward by Dr Horace Bushnell, he was conspicuous, using his influence to bring about harmony, and in the councils of the Congregational churches, over two of which, the Brooklyn councils of 1874 and 1876. he presided as moderator, he manifested great ability both as a debater and as a parliamentarian.
In his own theological views he was broad-minded and an advocate of liberal orthodoxy. In all matters concerning the welfare of his community or the nation, moreover, he took a deep and constant interest, and was particularly identified with the temperance and anti-slavery movements, his services to the latter constituting perhaps the most important work of his life. In this, as in most other controversies, he took a moderate course, condemning the apologists and defenders of slavery on the one hand and the Garrisonian extremists on the other. His Slavery Discussed in Occasional Essays from 1833 to 1846 (1846) exercised considerable influence upon Abraham Lincoln, and in this book appears the sentence, which, as rephrased by Lincoln, was widely quoted: "If that form of government, that system of social order is not wrong — if those laws of the Southern States, by virtue of which slavery exists there, and is what it is, are not wrong — nothing is wrong."
He was early attracted to the study of the ecclesiastical history of New England and was frequently called upon to deliver commemorative addresses, some of which were published in book and pamphlet form. Of these, his Thirteen Historical Discourses (1839), dealing with the history of New Haven, and his Four Commemorative Discourses (1866) may be especially mentioned. The most important of his historical works, however, is his Genesis of the New England Churches (1874). He published A Manual for Young Church Members (1833); edited, with a biography, the Select Practical Writings of Richard Baxter (1831); and was the author of a number of hymns, the best-known of which is the one beginning, "O God, beneath Thy guiding hand Our exiled fathers crossed the sea."
There is much biographical material in the commemorative volume issued by his congregation, Leonard Bacon, Pastor of the First Church in New Haven (New Haven, 1882), and there is a good sketch in Williston Walker's Ten New England Leaders (New York, 1901).
Leonard Bacon's son, Leonard Woolsey Bacon (1830-1907), graduated at Yale in 1850, was pastor of various Congregational and Presbyterian churches, and published Church Papers (1876); A Life Worth Living: Life of Emily Bliss Gould (1878); Irenics and Polemics and Sundry Essays in Church History (1895); History of American Christianity (1898); and The Congregationalists (1904).