Whittier is depicted so often as the gentle hoary-headed Quaker that the fiery politician within him is often forgotten. He declared himself an abolitionist in the pamphlet Justice and Expediency (1833) and went to the unpopular national antislavery convention. In 1834–35 he sat in the Massachusetts legislature; he ran for Congress on the Liberty ticket in 1842 and was a founder of the Republican party. He also worked staunchly behind the political scene to further the abolitionist cause and was an active antislavery editor until 1840, when frail health forced him to retire to his Amesbury home.

From there he sent out more of the poems and essays that made him a spokesman for the cause, and he was corresponding editor (1847–59) of the Washington abolitionist weekly, the National Era. In addition, Whittier compiled and edited a number of books; the most entertaining was the semifictional Leaves from Margaret Smith's Journal (1849). Meanwhile, his volumes of verse came out almost biennially; the first authorized collection appeared in 1838.

After the Civil War he turned from politics and dedicated himself completely to poetry. Although he liked to think of himself as the bard of common people, as in Songs of Labor (1850), his best work is his careful and accurate delineation of New England life, history, and legend. His most famous poem is Snow-bound (1866), an idyllic picture of his boyhood home; other memorable volumes are The Tent on the Beach (1867) and Maud Muller (1867). Such ballads as “Barbara Frietchie,” “Marguerite,” and “Skipper Ireson's Ride”; perennial favorites like “The Barefoot Boy” and the war poem “Laus Deo”; and his nearly 100 hymns, of which the best known is “Dear Lord and Father of Mankind,” gave him popularity in his time surpassed perhaps only by Longfellow.

In current critical estimation, Whittier's ability as a balladist surpassed his ability as a poet. His meters and rhythms were conventional and his poems tended to be too profuse. Nevertheless, as the voice of the New England villager and farmer prior to industrialization, his work portrays an important period in American history.