Cominar manages a portfolio consisting of 430 office, retail, and industrial properties, totalling 38.4 million square feet. These properties are located in Quebec and Ontario. As of August 2018, Cominar REIT had assets of C$6.9 billion (Canadian dollars). Cominar has been described as one of the largest commercial property owners in Canada and is ranked as the largest commercial property owner in Quebec.
Cominar was founded in 1965 by Jules Dallaire. The company was originally focused on building apartments in Quebec City. In 1973, it expanded into the construction of condominiums.
In 1986, Cominar developed Place de la Cité in Sainte-Foy, a borough of Quebec City, an office tower set above a shopping concourse.
In 1998, Cominar was listed on the Toronto Stock Exchange as a FPI Cominar (where FPI stands for Fonds de placement immobilier; a unincorporated closed-end investment trust). At the time, Cominar had 51 properties totalling 3.1 million sq.ft. of leasable space, and claimed to be the first fully-integrated real estate investment trust in Canada.
In 2009, Cominar completed a public offering of REIT units for a total of $575M (Canadian dollars).
In 2012, Cominar purchased 68 properties representing 4.3 million sq.ft. from GE Capital Real Estate (Canada). In 2014, it announced the purchase of $1.35 Billion (Canadian dollars) of properties (a total of 5 million sq.ft.) from Ivanhoé Cambridge, a real estate subsidiary of the Caisse de dépôt et placement du Québec. 
In August 2017, following a downgrading of its credit rating by the global credit rating agency DBRS, Cominar announced the divestment of 14% of its holdings for a total of 100 properties. In December 2017, Cominar announced the sale of 97 properties for $1.14 billion (Canadian Dollars). As a result of the sale, the company no longer owned properties in Western Canada, in the Toronto area or the Atlantic provinces. Overall, Cominar's property holdings were limited to Ontario and Quebec. At about the same time, Cominar's CEO, Michel Dallaire, was replaced by Sylvain Cossette; Dallaire remained chairman and a major shareholder.
- "Cominar changes chief executive as REIT repositions it operations". Financial Post. 2017-10-04. Retrieved 2018-08-23.
- "Learn About Our Real Estate Investment Trusts | Cominar". cominar.com. Retrieved 2018-08-22.
- "Cominar REIT to focus on Ottawa, Quebec as it sells 14% of portfolio | Ottawa Business Journal". www.obj.ca. Retrieved 2018-08-24.
- "Lexpert ® | Cominar REIT Completes $575M Public Offering". Retrieved 2018-08-29.
- "Cominar REIT may buy 68 properties from GE Capital with proceed from public offering". Financial Post. 2012-08-21. Retrieved 2018-08-29.
- "Cominar Plans to Concentrate on its Core Markets and to Reduce Debt Level". Newswire. August 22, 2017. Retrieved 20 September 2018.
- "Cominar REIT sells 97 non-core properties to Slate Acquisitions for $1.14 billion". Financial Post. 2017-12-18. Retrieved 2018-08-24.
- "Cominar changes chief executive as REIT repositions it operations". Financial Post. October 4, 2017. Retrieved 20 September 2018.