Douglas began practicing law in 1834, followed quickly by political ventures, including the office of Illinois attorney general. In 1840, Douglas became Illinois secretary of state, then served as a judge on the state supreme court from 1841 to 1843. He was elected to the House of Representatives in 1843 and to the Senate in 1847.
Douglas played a major role in most of the major public issues of his day. He was an ardent expansionist, advocating the annexation of Cuba and the entirety of the Oregon Territory. In the Senate Douglas chaired the influential Committee on Territories. With Henry