 Initially elected to U.S. House of Representatives in 1850 as a Unionist, Hillyer was re-elected in 1852 as a Democrat, and he served from March 4, 1851 to March 3, 1855.In his second term Hillyer was chairman of the Committee on Private Land Claims.After his congressional career, Hillyer was appointed by President James Buchanan as Solicitor of the United States Treasury, and served from 1857 until February 13, 1861 when, as a result of Georgia's seccession from the Union, he resigned his post and returned to Georgia.