MacKay was born in New Glasgow, Nova Scotia, the son of PC cabinet minister Elmer MacKay.
He was first elected to the Canadian House of Commons on June 2, 1997 for Pictou-Antigonish-Guysborough, a riding in northeastern Nova Scotia and served as Justice Critic and House Leader.
MacKay won the party leadership on May 31, 2003 after a deal with leadership candidate David Orchard. Mackay promised a review of the NAFTA free trade agreement, and also promised that no deal on electoral cooperation would be made with the Canadian Alliance. He has since broken this deal, and worked with the Canadian Alliance to merge the two parties into one, the Conservative Party of Canada. MacKay's efforts to sell the merger to the PC membership were successful, as 90.4% supported the deal in a vote on December 6, 2003. The PC caucus, however, had some holdouts, including long-time Tory MP and former Prime Minister Joe Clark and now-turned-Liberal MP Scott Brison.
MacKay announced on January 13, 2004 that he would not run for the leadership of the new Conservative Party of Canada.
His companion is Lisa Michelle Merrithew, daughter of former Conservative party Cabinet Minister (Veterans Affairs) (1984-1993), Gerald Stairs Merrithew.