[1] After attending Phillips Exeter Academy, Sewall entered Harvard College at the age of 13, graduating in 1817 near the top of his class.

Later he helped fund the Liberator, Garrison's abolitionist newspaper,[7] and co-founded the Massachusetts Anti-Slavery Society,[8] joining the Board of Managers in 1832.

[9] Sewall volunteered his services as a lawyer to the Society, drafting petitions, resolutions, arguments, and legal defenses, as well as preparing annual reports and writing articles for the Liberator.

"[12] On August 1, 1836, Sewall represented Eliza Small and Polly Ann Bates, two fugitives from Baltimore who had been held prisoner by the captain of the Chickasaw.

Some commentators suspected Sewall of instigating the riot; he received threatening letters and was physically assaulted in his office by a relative of the slaveholder.

Loring and Sewall argued in Commonwealth v. Aves that the Massachusetts constitution banned slavery and that therefore the child was freed as soon as she entered the state.

In another case, that of a girl named Amy who had been brought to Boston from New Orleans, Sewall was unsuccessful; the judge allowed the slaveholder to leave with the child on the grounds that she "appeared happy and contented and was acting under no visible restraint.

[16] In 1848, he joined a litigation team in Washington, D.C. that included Francis Jackson, Salmon P. Chase, Samuel Gridley Howe, Horace Mann, and Robert Morris.

[17] Together with Ellis Gray Loring, Robert Morris, and Richard Henry Dana Jr., Sewall filed a petition for writ of habeas corpus calling for Minkins's release from police custody.

When the petition was denied, members of the Boston Vigilance Committee rescued Minkins, who escaped to Canada with help from the Underground Railroad.

[18] That same year, Sewall was one of the defense counsel (along with Robert Rantoul Jr. and Charles Greely Loring) of Thomas Sims, a fugitive slave from Savannah, Georgia.

During the meeting, a small band of abolitionists led by Thomas Wentworth Higginson broke down the courthouse door with a battering ram in an attempt to free Burns.

He introduced a bill which he claimed was the shortest ever enacted by the Massachusetts legislature: "Aliens may take, hold, convey, and transmit real estate.

[27] Sewall encouraged women reformers, such as Lucretia Mott and Sarah and Angelina Grimké, who were criticized for speaking in public.

Marie Elizabeth Zakrzewska, a pioneering woman physician who headed a department there, later said of Sewall, "He served the cause for the education of medical women when this was so unpopular as to call forth ridicule upon any man who openly avowed it."

[30] He frequently appeared before the state legislature with Lucy Stone and Henry Browne Blackwell to press for reforms, and sent regular updates to the Woman's Journal.

When Massachusetts women were granted the right to vote in school committee elections, he published a pamphlet of instructions for the novice voters.

[31] In the summer of 1835, while attending an anti-slavery conference in New York, Sewall met a Quaker family, Nathan and Comfort Winslow of Portland, Maine, and their daughter Louisa.

[35] The poet John Greenleaf Whittier, Sewall's lifelong friend and fellow abolitionist, wrote a poem in his memory.