John Henry "Jack" Horner PC (July 20, 1927 – November 18, 2004) was a Canadian politician. He was a Member of Parliament (MP) in the House of Commons of Canada for the electoral areas of Acadia and Crowfoot. He was an MP from 1958 to 1979. Horner is famous for fighting very hard for ranchers and farmers.
When he was in politics, Horner was very well-known for being a right-wing politician. He strongly supported the Progressive Conservative Prime Minister from 1957 to 1963, John Diefenbaker. Many of his family were very important in Western Canadian politics. They were all part of a group called "Diefenbaker's cowboys". Horner tried to become the leader of the Progressive Conservative Party many times. After he lost to Joe Clark in trying to become the leader in 1976, he crossed the floor, or changed the political party he was part of. He went from the Progressive Conservative Party to the Liberal Party.
After he became a Liberal, Pierre Trudeau made him a cabinet minister. He became a Minister without portfolio, then the Minister of Industry, Trade, and Commerce. He lost his election in 1979. He later became a part of the board of directors of the Canadian National Railway.