Simon Greenleaf (December 5, 1783 – October 6, 1853), American lawyer and jurist, was born at Newburyport, Massachusetts.
Greenleaf's family traces its ancestry back to Edmund Greenleaf, who lived in Ipswich, Suffolk in England before emigrating and settling in Newburyport, Massachusetts. The Greenleaf family flourished in this part of Massachusetts for almost 150 years prior to Simon's birth in 1783. Simon's father, Moses Greenleaf, married Lydia Parsons, daughter of Rev. Jonathan Parsons of Newburyport. Their son Moses Greenleaf (1777–1834), Simon's older brother, became a distinguished surveyor and mapmaker in Maine.
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).