He served two terms as a Democrat (1831–1835). He served in the American forces during Revolutionary War and later moved to Kentucky. In 1806 he moved to Louisiana.
He commanded troops who on 1810 September 23 captured the Spanish fort in at Baton Rouge, commencing the West Florida Rebellion of 1810.
A few days later the West Florida Assembly, meeting at Saint Francisville, commissioned General Philemon Thomas to march the West Florida Army across the newly proclaimed Republic. According to Southeastern Louisiana University history professor Sam Hyde,
Philemon Thomas served in the War of 1812 and later served two terms in Congress.
He ran for Governor twice, in 1824 and in 1828. He died in Baton Rouge and is interred there in the Baton Rouge National Cemetery.
He was a member of Kentucky"s Constitutional Convention and served in the state House and state Senate.