Jonathan Sewall began his working life as a teacher in Salem before entering the law and establishing a practice in Charlestown, Massachusetts. His legal ability and official connections brought appointments in 1767 as solicitor general, attorney general and advocate general in admiralty. He also received a vice-admiralty judgeship associated with Nova Scotia. These offices placed him within the crown's legal establishment as disputes over colonial rights and imperial authority intensified.

Sewall was a close friend of John Adams and attempted to draw him into royal legal service. Their political paths diverged as Adams entered the resistance movement and Sewall remained committed to the British connection. In September 1774 a crowd attacked Sewall's Cambridge home, and his family moved into Boston for protection. He left Massachusetts for England in 1775 and was subsequently included in the state's banishment legislation. In exile he adopted the spelling Sewell, which his sons later used.

Sewall met Adams again in London in 1785, when Adams was serving as the American minister. Their conversation revived a personal connection without undoing their opposed judgments of the Revolution. Sewall later settled in British North America and continued work connected with vice-admiralty jurisdiction. He died at Saint John, New Brunswick, in 1796. The later chief justiceship of Lower Canada belonged to his son Jonathan, not to the former Massachusetts attorney general.