Philemon Beecher was born in Oxford, Connecticut, the son of Abraham Beecher and Desire Tolles. Philemon received a classical education, read law and was admitted to the bar.

Philemon Beecher moved to Lancaster, Ohio, in 1801 and continued the practice of law, being admitted to the bar while Ohio was still the Northwest Territory. He was the leading lawyer of the Lancaster bar for twenty-five years. It was in his office that the great lawyer and political figure Thomas Ewing studied law. Beecher was often a barrister at the court house in Marietta, Ohio.

Beecher was a member of Scioto Lodge No 2 and senior Deacon of the Grand Lodge of Ohio of Free and Accepted Masons. Philemon Beecher made the acquaintance of Susan Gillespie, a daughter of Neil Gillespie of Brownsville, Pennsylvania, when she came to Lancaster on a visit to her sister, Mrs. Hugh Boyle. Philemon Beecher and Susan Gillespie were married in Pennsylvania in 1803 or 1804.

Originally a Federalist, Beecher was elected a member of the Ohio House of Representatives in 1803 and again in the three sessions from 1805 to 1807, serving as speaker in 1807. His swarthy complexion earned him the sobriquet of the "Black Knight." In 1805, he opposed a Resolution commending the United States government for the Louisiana Purchase, but the Resolution passed by one vote. Beecher was a witness on behalf of Fairfield County, Ohio Common Pleas Judge William W. Irvin at the latter’s impeachment trial in 1806. Irvin was married to the sister of Beecher’s future wife. Irvin went on to follow Beecher in the Ohio legislature, in Congress, and was later on the Ohio Supreme Court.

Philemon Beecher was unsuccessful as candidate for United States Senator in 1807 against Edward Tiffin. That year he also failed to win a place on the bench as judge of the Ohio Supreme Court. Philemon Beecher was appointed a major general in the Ohio militia.

Lancaster, Ohio, Bank was chartered in 1816 with Philemon Beecher as president for one year. He continued as a director of the bank for many years.

Philemon Beecher was elected as a Democratic-Republican from Ohio's 5th congressional district to the Fifteenth and Sixteenth Congresses. He was an unsuccessful candidate for reelection in 1820.

Described in later years as an old-line Whig, in 1822, Philemon Beecher was elected as an Adams-Clay Republican from the new Ohio 9th district to the Eighteenth Congress, and an Adams candidate to the Nineteenth and Twentieth Congresses. In the 1824 battle between Andrew Jackson and John Quincy Adams for Ohio’s electoral votes, he voted for Adams.

In 1826, Philemon Beecher participated as a managing member of The Colonization Society in Lancaster, an organization promoting repatriation to Africa as an alternative to slavery. He was an unsuccessful candidate for reelection in 1828 when he was defeated by his brother-in-law, William W. Irvin.

Philemon Beecher returned to Lancaster and continued the practice of law until his death there. He and his wife are interred in Elmwood Cemetery.