When Brian Mulroney became leader of the PC Party, Siddon was appointed the party's science critic in the shadow cabinet. After the Tories won the 1984 election, he was given the cabinet post of Secretary of State for Science and Technology. Siddon proved to be a hard working and competent MP, and was promoted to the Minister of Fisheries and Oceans position on November 21, 1985, in the wake of the tunagate scandal that had forced the resignation of previous minister, John Fraser.