Born near Lexington, Kentucky, Richardson attended Transylvania University, and then proceeded to teach school and study law.
He passed the bar exam in 1831 and started his practice in Shelbyville, Illinois. He was an attorney for the state from 1834 to 1835, and was elected representative to the state house, serving from 1836 to 1838. He moved over to the state senate from 1838 to 1842, and then back to the house again from 1844 to 1846, briefly serving as speaker of the lower house during his last term.
He was a presidential elector in 1844 for the Democrats.
Richardson enlisted as a captain in the United States. Army during the Mexican-American War, and was promoted to the rank of major. After the war, he moved to Quincy, Illinois, and then was elected to the 30th Congress to fill Stephen A. Douglas"s seat.
He was then reelected to the 31st, 32nd, 33rd, and 34th Congresses for the same seat (1847 to 1856). During his time in the House of Representatives, he was the Chairman of the Committee on Territories (32nd-33rd Congresses).
He resigned in August 1856 to run for Governor of Illinois.
He narrowly lost to fellow representative, and first nominee of the newly established Republican Party William H. Bissell. After being defeated, Richardson was appointed by President Buchanan as the Governor of the Nebraska Territory for most of 1858. Richarson resigned near the end of the year.
He was a delegate to 1860 Democratic National Convention from Illinois.
He then came back to Washington District of Columbia In 1863, he was elected to fill Stephen Douglas"s old seat in the United States Senate, defeating incumbent Republican Orville Browning. He was not renominated in 1865 and spent the rest of his life engaged in newspaper work.
He died on December 27, 1875 in Quincy, Illinois, where he is buried. Richardson County, Nebraska is named after him.
Member Illinois Ho; member Illinois Senate, 1838-1842. Member United States House Member United States Senate (filled vacancy) from Illinois, January