In the 1790s, Simon's parents moved to New Gloucester in
Maine, leaving him in Newburyport under the care of his grandfather Jonathan Greenleaf. There Simon was educated at the Latin school and studied the Greco-Roman classics. When he turned 16 years old, he rejoined his parents in New Gloucester. In 1801 he joined the law office of
Ezekiel Whitman (later the Chief Justice of Maine) and in 1806 was admitted to the Cumberland County bar as a legal practitioner. On 22 Mar 1806, in New Gloucester, he married Hannah Kingman. He then opened a legal practice at
Standish, but six months later relocated to
Gray, where he practised for twelve years, and in 1818 moved to
Portland. Greenleaf's
political preferences were aligned with the
Federalist Party, and in 1816 he was an unsuccessful candidate for that party in
Cumberland County for the Senate. He was reporter of the
Supreme Court of Maine from 1820 to 1832, and published nine volumes of
Reports of Cases in the Supreme Court of Maine (1820–1832). Greenleaf was elected a member of the
American Antiquarian Society in 1820. He was awarded the honorary
Doctor of Laws degree by Harvard in 1834. He received the same honor from Amherst in 1845, and again from the
University of Alabama in 1852. In 1848, he was elected as a member of the
American Philosophical Society. ==Professorships==