In 1836 Jackson entered politics representing Saline County, Missouri in the Missouri House of Representatives. Between 1838 and 1842 he was a cashier at the state bank in Fayette, Missouri. He was elected a state representative again in 1842, this time from Howard County, Missouri. In 1844 and 1846, he served as Speaker of the House. In 1848 he was elected to the Missouri State Senate. In 1852 he was elected to the House of Representatives again. From 1857 to 1860, he served as state bank commissioner. In 1860, as a Democrat, he secured his party's nomination for governor. He ran against Sample Orr in the general election and won. He was sworn in on January 3, 1861.