He joined the Labour Party in 1961 and several months later he became chairman of the Dunedin Central branch. In 1963 Phil Connolly, the retiring MP for Dunedin Central, shoulder tapped Mitchell to put his name forward to replace him in the seat. During their conversation Connolly was particularly concerned with what religion Mitchell was (assuming him to be a Catholic) and was relieved when Mitchell said he was an Anglican which would be acceptable to a predominantly Presbyterian constituency. However Mitchell ultimately did not put himself forward for the nomination, instead resolving to return to the UK.