The by-election held in Mid Ulster on 11 August 1955 was called as a result of a vote in the British parliament on 18 July 1955 which voted 197 votes to 63 to nullify the result of the previous 1955 UK General Election in the constituency. At that election, Sinn Féin candidate Tom Mitchell took the seat. In the by-election, Mitchell managed to retain the seat with an increase in the number of votes. This led to the 1956 Mid Ulster by-election.