After studying law, he began a successful practice in Charlestown and served as attorney general of Massachusetts from 1767 to 1775.

[2] In 1759 Sewall became a very close friend and patron of John Adams, the future second President of the United States.

At the urging of Governor Francis Bernard, Sewall offered Adams the position of Advocate General in the Admiralty Court.

While Adams was assigned to London as a U.S. minister to the Court of St. James's in 1785, he looked up his old friend and they had a two-hour meeting.

Both men were entrenched in their own ideas and no reconciliation was possible; Adams considered Sewall a casualty of the war.