Career
During his time in office he served as a cabinet minister in the government of Peter Lougheed from 1971 to 1975. Copithorne ran for a seat to the Alberta Legislature in the 1967 Alberta general election. On April 15, 1971 Copithorne joined the Progressive Conservative caucus giving up his independent status.
He ran for re-election in the 1971 Alberta general election as a Progressive Conservative candidate.
He increased his popular vote and held the district defeating two other candidates. After the election Premier Peter Lougheed rewarded Copithorne by appointing him to the Executive Council of Alberta to serve as the Minister of Highways and Transport.
Copithorne helped develop a plan to create Kananaskis Country in his district. He retired from the Alberta Legislature and his cabinet position at dissolution in 1975.