The firm was founded in 1973 by Roy Heenan, Donald Johnston, and Peter Blaikie as Johnston Heenan Blaikie. After Johnston left to become a Member of Parliament in 1978, the firm was renamed Heenan Blaikie. The firm was one of the first to expand across Canada in the 1970s, helping pioneer the concept of national law firms. It also established a reputation as the landing ground for former Prime Ministers: both Jean Chrétien and Pierre Trudeau joined the firm after their respective political careers ended.
At one point the home of over 500 lawyers, the firm began to suffer financial trouble in 2013. Hurt by falling revenues from a drop-off in business from resource companies, the opening of a Paris office at a time the economy was struggling there, and the end of several major cases at the same time, the firm announced a drop in income per partner of 10 to 15 percent. Only week later the firm faced a rash of defection, with nearly 30 senior partners exiting for more profitable firms. The firm voted on February 5, 2014 to start the orderly dissolution of the firm, the largest in Canadian history, surpassing the previous record set by Goodman and Carr in 2007.
On February 28, 2014, the firm closed its doors, leaving a skeleton crew of support staff to wind down the firm's operations over the next few months.
At the international level, Heenan Blaikie was counsel to the Canadian Employers Council, an organisation that represents Canadian employers at the International Labour Organization (ILO). Heenan Blaikie was also the Canadian member of the National Workers' Compensation Defense Network (NWCDN), an organization of independent law firms in the United States and Canada with an established workers' compensation practice and experience in defending employers and insurance companies in workers' compensation and related actions. In addition to its European and Asian presence, its Paris office served as a foray into the African market. However, the firm's activities in Africa have garnered controversy, as its endeavours there included a working relationship with Ari Ben-Menashe, from whom the firm has since tried to distance itself.