During the October Crisis of 1970, he agonized over the implementation of the War Measures Act, and was prepared to vote against it, but relented for the sake of keeping the Tory caucus united behind Robert Stanfield. Writing in retrospect, MacQuarrie described his vote in favour of the Act as "fundamentally wrong".
He was first elected to the Canadian House of Commons as a Progressive Conservative candidate in the 1957 federal election that brought John Diefenbaker to power. He served as a Member of Parliament for twenty-two consecutive years, until he was elevated to the Canadian Senate in 1979 on the advice of Joe Clark. He sat in the upper house for a further fifteen years, retiring at the mandatory age of 75 in 1994.
He was publicly loyal to Clark's successor, Brian Mulroney, but privately disagreed with the government on several occasions, once saying during a caucus meeting, "You know, a lot of people think I have a prominent nose because of my enjoyment of a certain beverage. Well, that's all nonsense. I got it that way by having to hold it so often while voting for some of Mulroney's bills."
MacQuarrie was educated at Prince of Wales College, the University of Manitoba, the University of British Columbia and McGill University. He earned his doctorate at McGill, choosing for his thesis topic Robert Borden. He lectured at Brandon University and at Mount Allison University, in economics, political science and international relations.
He edited and wrote the introduction to the published edition of Sir Robert Borden's diaries. An admirer of the World War I-era prime minister, MacQuarrie considered Borden to be the architect of Canadian independence.
Heath MacQuarrie died at the age of 82. He was survived by his wife, Isabel and three children: Heather, Flora and Iain.