Career
After practicing law for a time in Pittsburgh, Pennsylvania, in 1852 Shannon ran unsuccessfully for a seat in the United States House of Representatives (as a Democrat), losing by five hundred votes to Whig David Ritchie. Shannon was then appointed president judge of the local district court in 1852 by Pennsylvania Governor William Bigler, but served for only a year, having been defeated for re-election in 1853. At the beginning of the Civil War Shannon became a Republican and was elected to the Pennsylvania House of Representatives, serving two terms, 1862 and 1863.
In 1862 he campaigned with Governor Andrew Curtin in his successful re-election bid, and in 1864 was part of Lincoln"s re-election campaign in Pennsylvania.
After the Civil War Shannon continued his law practice until he was appointed Chief Justice of the Supreme Court of the Dakota Territory in 1873 by President Ulysses South. Grant. Shannon presided over the trial of Jack McCall for the killing of Wild Bill Hickok which resulted in the Dakota Territory"s first legal hanging, and prepared the Criminal Code adopted by the Territorial Legislature in 1875.
President Chester A. Arthur appointed him to a commission negotiating land sales with the Sioux, which he served on for a number of years. He signed an 1882 agreement with the Lakota which was never ratified.
In 1883 Shannon was appointed to investigate charges of drunkenness and improper behavior against Everton Conger, a judge in Montana Territory.
Although his report was somewhat sympathetic, it led to Attorney-General Benjamin H. Brewster recommending his removal. Conger was ultimately restored to office by President Arthur, but only one day before the expiration of his commission. Shannon testified before a Congressional committee about his investigation in March 1884.
Shannon moved to California only a year or two before his death, due to ill health, and died after being injured in a carriage accident.
Shannon County, South Dakota was named in his honor until May 1, 2015 when it was renamed Oglala Lakota County.