John Henry "Jack" Horner PC (July 20, 1927 – November 18, 2004) was a Canadian rancher, politician, and Cabinet minister.
His father, Ralph Horner, was a failed Conservative candidate who was appointed to the board of directors of Canadian National Railways by the government of R. B. Bennett in 1931, and then to the Senate of Canada in 1933.
At the 1966 Tory convention which changed the rules to allow a challenge to a sitting leader, Horner threw a punch at Dalton Camp supporter Roy McMurtry, and accosted Brian Mulroney in a hallway.
Horner worked to undermine Stanfield's leadership through manoeuvers such as leading a revolt against the party's support for the Official Languages Act.
He finished fourth in the contest, and threw his support to Claude Wagner, who lost on the final ballot to Joe Clark.
On April 20, 1977, Horner shocked his constituents and many political observers by crossing the floor to join the Liberal Party, which was at the time deeply unpopular in Alberta.
When Diefenbaker, who was still in Parliament 14 years after leaving the Prime Ministership, learned of Horner's defection, he said, "the sheriff has joined the rustlers."
Although it retained the name of Crowfoot, most of its constituents came from neighbouring Battle River, represented by fellow Tory Arnold Malone.
Many critics argued that Horner's floor crossing was motivated by opportunism as opposed to ideology and ultimately provoked by the pending electoral redistribution.