He was born in Salem, Massachusetts, to Mitchel Sewall and Elizabeth Price.
[1][2][3][4][5][6] (or according to other sources, York in 1749[7] but this is not correct as on Sunday, March 27, 1748 he was christened Jonathan Mitchel Sewall just along the street from his parents home in Salem at the First Church of Salem by The Rev.
He studied law with his cousin Jonathan Sewall (1729-1796) in Cambridge, Massachusetts.
He joined the law office of John Pickering (1737 - 1805) in Portsmouth, New Hampshire in the early 1770s.
In 1774 he became register of probate in Grafton County, New Hampshire but resigned the following year.