He took part in an unsuccessful IRA raid on a British Army barracks in Omagh, County Tyrone in October 1954,[2] and as a result received a sentence of 10 years imprisonment for treason felony.
[3] While serving his sentence in Crumlin Road prison, Mitchell was nominated as a Sinn Féin candidate on an abstentionist platform for the Mid-Ulster constituency in the May 1955 UK general election.
The Forfeiture Act 1870 provided that anyone convicted of treason or felony and sentenced to a term of imprisonment exceeding 12 months was incapable of being elected to or sitting in the House of Commons.
[4] On 18 July 1955 a resolution of the House of Commons, passed by 197 votes to 63, formally declared that Mitchell was covered by this provision, vacated his seat, and ordered that a by-election be held.
Mitchell and O'Neill split the nationalist vote and he was defeated at the polls on 8 May 1956 by George Forrest, who stood as an Independent Unionist candidate.