The idea to build the Forum in 1923 is credited to Sir Edward Wentworth Beatty, president of the Canadian Pacific Railway. At the suggestion of Senator Donat Raymond, William Northey developed a plan for a 12,500 seat capacity rink. Plans were scaled back for financial reasons to a rink of 9,300 seats. Even at the reduced size, the rink could not immediately find financing. The Forum would eventually be financed by H. L. Timmins. The site selected was the site of a roller skating rink named the Forum, and the name was kept. The site had previously been the site of an outdoor ice hockey rink, used by Frank and Lester Patrick, Art Ross and Russell Bowie as youths.
The Forum opened on November 29, 1924, at a total cost of C$1.5 million ($22.8 million in 2020 dollars) with an original seating capacity of 9,300. It underwent two renovations, in 1949 and 1968. When the Forum closed in 1996 it had a capacity of 17,959, which included approximately 1,600 in standing room.
By the time of the 1968 renovations, a centre hanging digital scoreclock was installed, designed by the Day Sign Company of Toronto and similar to those installed at the Boston Garden and Chicago Stadium during the 1970s. A new centre hanging scoreclock, designed by Daktronics, was installed in 1985 and contained on each side a color matrix board. Along with one other Original Six indoor ice hockey arena, the Boston Garden, the Montreal Forum used a high-pitched siren to signal the end of an NHL game's period — the siren would later be re-installed in the Forum's successor facility, the Bell Centre (and still in use there), much as the TD Garden in Boston inherited the lower-pitched Garden's siren. A Rainforest Cafe was planned to open at the location following its closure, but was never built.