Initially, the Reserve Army was known as the N.P.A.M. (Non Permanent Active Militia), and provided internal security by guarding various vulnerable areas. Eventually, the RCMP (Royal Canadian Mounted Police) took over these tasks, and the emphasis of the Reserve Army was placed on providing a proportion of replacements for units in the Active Army. 2nd (Reserve) Battalions of active Regiments were mobilized, to provide trained recruits for the Active Battalions. In December 1941, when Japan joined the war, the public became increasingly concerned with the threat posed to Canada itself. The Reserve Army was now given an expanded role. It would provide a trained reserve for the Active Army in the defence of Canada, as well as forming a basis for expansion of the Active Army should the situation warrant, plus it would help out the civil authorities if needed (internal policing).

The majority of enlistment's into the Reserve Army, were those not eligible for the Active Army. Those 19-35 with a medical category lower than "B", those 17-19, and older than 35, and those who were excluded from obligatory service under the N.R.M.A. (National Resources Mobilization Act). In addition, all personnel in the Canadian Officers Training Corps, would be assigned to the Reserve Army, prior to their graduation.

On 31 December, 1941, 8 Reserve Brigade Groups were authorized, and hastily trained for home defence. In February 1942, 4 more Reserve Brigade Groups were authorized, so that each of the 11 Military Districts would have one Brigade Group, with Military District No.6 (Nova Scotia and P.E.I.) having two. For obvious reasons, the Active Army had first priority on equipment and weapons, but as more and more equipment was coming off the production lines, the Reserve Army became increasingly better equipped. The Reserve Brigade Groups were given full-time commanders and staffs, and although it was of secondary importance next to the Active Army, some invaluable contributions were made by the Reserve Army in the security of the lower St.Laurent river. In mid 1942, when the 'threat' to the east coast was at its height, the local population along the St.Laurent became alarmed after U-boats began sinking ships in the Gulf of St.Laurent, and the river itself. Instead of tying down thousands of troops, National Defence Headquarters insisted that security be provided by the local communities. An aggressive recruiting campaign began in September 1942 along the Gaspé Peninsula, and by February 1943, 2 (Reserve) Battalions of Les Fusiliers du St.Laurent had been raised, and provided local security measures, such as manning road-blocks, enforcing black-outs, as well as providing personnel for various telephone detachments, and did much to ease tensions of the local populace.

The peak strength of the Reserve Army, was reached in early December 1940, when the total was at 111,579. After then, the numbers steadily decreased, until in February 1942, when strength was at 69,660. By April 1945, the total had risen to roughly 105,000.