Mitchell began his political involvement when he supported Seán MacBride, leader of the radical republican Clann na Poblachta at the 1957 general election. He joined Fine Gael in 1967, becoming that party's unsuccessful candidate in a by-election in 1970. He sought a party nomination to run at the 1973 general election. However, he agreed not to contest the seat to allow Declan Costello, a senior figure in his party and son of former Taoiseach John A. Costello, to be elected. Costello went on to serve as Attorney General of Ireland in the 1973–1977 National Coalition of Fine Gael and the Labour Party.