After practicing law for a time in Pittsburgh, Pennsylvania, in 1852, Shannon ran unsuccessfully for a seat in the US House of Representatives (as a Democrat), losing by five hundred votes to Whig David Ritchie.
[1] 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.
[3] 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 S. Grant.
He was reappointed by President Rutherford B. Hayes, but his application for a third appointment was opposed by the Yankton Bar Association, perhaps in part because he was viewed as a supporter of Governor Nehemiah Ordway.
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.