The pre-war Canadian Army consisted of the Permanent Force and the Non-Permanent Active Militia (N.P.A.M.). Each became part of the Canadian Active Service Force (C.A.S.F.) when mobilised. Most of the cavalry regiments formed tank regiments, armour regiments, reconnaissance regiments, and armoured car regiments, when they became C.A.S.F. The N.P.A.M. and Permanent Force on September 3, 1939 was organised into eleven military districts organised geographically and numbered 1-7 and 10-13. Within the districts existed brigades, also organised geographically. The infantry brigades were numbered 1-19 while the cavalry brigades were numbered 1-4 and 7. The pre-war brigades had no bearing on those mobilised for service under the C.A.S.F. and went out of use shortly after the start of war.

Certain units were mobilised on 26 August 1939 in anticipation of the declaration of war. They were mostly coast defence formations. Many militia units also mobilised details to guard vulnerable points. For example, the Regina Rifle Regiment called out a guard of four officers and 44 other ranks to guard Regina airport, an armoury guard of a warrant officer and 12 other ranks and an ordnance guard of one NCO and six other ranks. The following units mobilised on 26 August 1939 and became C.A.S.F details on 1 September 1939:

1st (Brighton) Fortress (Electrical and Mechanical) Company - Saint John, New Brunswick
2nd Fortress (Electrical and Mechanical) Company - Halifax, Nova Scotia
3rd Fortress (Electrical and Mechanical) Company - Sydney, Nova Scotia
4th Fortress (Electrical and Mechanical) Company - Vancouver, British Columbia
1st (Halifax) Coast Brigade, RCA (51st, 52nd, 53rd, 9th Hvy, 1st AA, 9th, 10th S/L Btys) - Halifax, NS
3rd (New Brunswick) Coast Brigade, RCA (4th Hvy, 1st S/L Btys) - Saint John, New Brunswick
5th (British Columbia) Coast Brigade, RCA (55th, 56th, 60th Hvy, 2nd AA, 17th S/L Btys) - Victoria, BC
15th (Vancouver) Coast Brigade, RCA (31st, 58th Hvy, 3rd S/L Btys) - Vancouver, BC
16th Coast Brigade, RCA (6th, 36th, 86th Hvy Btys) - Sydney-Canso, Nova Scotia
5th Heavy Battery, RCA (Permanent Force) - Esquimalt, BC
1st Anti-Aircraft Regiment, RCA (9th, 10th, 11th AA Btys) - Vancouver, BC - Details only
In addition ten infantry battalions, with HQs near the coast had details active. These details remained active until each was mobilized on 1 January 1941:

The Cape Breton Highlanders - Sydney, Nova Scotia
The Halifax Rifles - Halifax, Nova Scotia
The Irish Fusiliers of Canada - Vancouver, BC
The New Brunswick Rangers - Sussex, New Brunswick
The Pictou Highlanders - Stellarton, Nova Scotia
The Prince Edward Island Highlanders - Charlottetown, Prince Edward Island
The Princess Louise Fusiliers M-G - Halifax, Nova Scotia
The Rocky Mountain Rangers - Kamloops, British Columbia
The St. John Fusiliers M-G - St. John, New Brunswick
2nd Battalion, The Canadian Scottish - Nanaimo, British Columbia
The Canadian Active Service Force was mobilised on 1 September 1939 under General Order 135. It consisted of the 1st and 2nd Canadian Infantry Divisions along with supporting units for an army corps. It was formed from elements of the Permanent Force and mobilized militia units.

1st Canadian Infantry Division HQ was formed in October 1939. The 1st Canadian Infantry Brigade was raised in Ontario. Its battalions remained at their home stations until November 1939, when the brigade assembled at Valcartier, PQ. It embarked for the United Kingdom on 17 December 1939 and arrived at Greenock, Scotland on 25 December. It moved to Aldershot on arrival. The 2nd Canadian Infantry Brigade was raised in Western, Canada. The battalions remained at their home stations until they moved to Halifax in mid-December 1939 to depart. 2nd Brigade embarked for the United Kingdom on 22 December 1939 and arrived at Greenock, Scotland on 30 December. It moved to Aldershot on arrival. The 3rd Canadian Infantry Brigade was raised in Quebec and the Maritimes. The battalions remained at their home stations until early December 1939, when they moved to Halifax. It embarked for the United Kingdom on 8 December 1939 and arrived at Greenock, Scotland on 17 December. It also moved to Aldershot on arrival. The tank and cavalry regiments associated with the division remained in Canada as did most of the corps units.

2nd Canadian Infantry Division was raised on 1 September 1939 in Canada, but the units remained at their mobilization HQs until the spring of 1940. Recruiting for the division was suspended in October 1939. Recruiting for specialists resumed on 18 February and for the rest of the division on 18 March. The divisional HQ was not organized until the end of May 1940 in Ottawa. The 4th Canadian Infantry Brigade was raised in Ontario, Canada. The battalions remained at their mobilization HQs until the brigade assembled at Camp Borden, Ontario on 3 June 1940. [The Royal Regiment of Canada left 9 June 1940 for Halifax and then served on Iceland from 16 June 1940 to 31 October 1940. It arrived at Greenock, Scotland on 3 November 1940 and rejoined the brigade at Aldershot. The Cameron Highlanders of Ottawa left Canada on 30 June 1940 and arrived in Iceland on 7 July 1940, where it remained until 27 April 1941.] The rest of the brigade embarked for the United Kingdom on 23 July 1940 and arrived at Gourock, Scotland on 2 August. It moved to Aldershot on arrival. The 5th Canadian Infantry Brigade was raised in the Province de Quebec. The battalions remained at their mobilization HQs until the brigade assembled at Valcartier, PQ on 25 May 1940. [1st Black Watch was sent to Newfoundland on 21 June 1941 and remained there until 11 August 1940. It rejoined the brigade at Halifax on 13 August. Les Fusiliers Mont-Royal left the brigade on 7 July 1940 and arrived in Iceland on 9 July 1940. It officially left the brigade on arrival in the United Kingdom] The rest of the brigade, with the Calgary Highlanders, embarked for the United Kingdom on 27 August 1940 and arrived at Gourock, Scotland on 4 September. It moved to Aldershot on arrival. 6th Canadian Infantry Brigade was raised in Western Canada. The battalions remained at their mobilization HQs until the brigade assembled at Camp Shilo, Manitoba in May 1940. [The Winnipeg Grenadiers left the brigade in late May 1940 for service in the Caribbean. The Calgary Highlanders left the brigade in June 1940 for Valcartier to join 5th Brigade and replace Les Fusiliers Mont-Royal.] The two battalions remained at Camp Shilo, until it was time to leave for the United Kingdom. They embarked on 16 December 1940 and arrived on 25 December 1940 in the United Kingdom. It moved to Aldershot on arrival and was joined there by Les Fusiliers Mont-Royal from Iceland on 3 November 1940.

The fall of France in May 1940 had an effect on efforts to raise additional units in Canada. The authorization for a further corps of two divisions was given on 17 May. Units were mobilised from May 24, 1940 although the call out list was not published until September 5, 1940. The 3rd Canadian Infantry Division was formed on 5 September 1940 and concentrated in the Maritimes at Camp Debart, Nova Scotia and Camp Sussex, New Brunswick. 7th Canadian Infantry Brigade was raised on 24 May 1940 in as a Scottish brigade. The battalions remained at their RHQs until the brigade assembled at Camp Debart, Nova Scotia in October 1940. There were changes in the brigade organization before it assembled in Nova Scotia. It left Halifax on 24 August 1941 and arrived in Gourock, Scotland on 1 September 1941. It moved to Aldershot on arrival. 8th Canadian Infantry Brigade was raised on 24 May 1940. The battalions also remained at their RHQs until the brigade assembled at Sussex, New Brunswick in early December 1940. There were some changes in the brigade organization before it assembled in New Brunswick. It left Halifax on 21 July 1941 and arrived in Gourock, Scotland on 29 July 1941. It also moved to Aldershot on arrival. 9th Canadian Infantry Brigade was raised on 24 May 1940 and battalions remained at their RHQs until the brigade assembled at Camp Debart, Nova Scotia in early March 1941.Some changes in the brigade organization took place before it assembled in New Brunswick. It left Halifax on 21 July 1941 and arrived in Gourock, Scotland on 29 July 1941, moving to Aldershot on arrival. 4th Canadian Infantry Division was authorized on 27 May 1940. The division was not established until 10 June 1941 at Camp Debart, Nova Scotia. It remained in the Maritimes until converted to 4th Canadian Armoured Division on 26 January 1942. Its battalions were located at Niagara-on-the-Lake, Camp Borden, Valcartier, Sussex and Camp Debart. 10th Canadian Infantry Brigade assembled at Naniamo, British Columbia in October 1940. The brigade HQ was not established until February 1941 at Naniamo. It was replaced in April 1941 by 13th Brigade and moved to the east in the Niagara area, where it took over the duties of 13th Brigade. It was converted to 3rd Canadian Armoured Brigade on 26 January 1942. The battalions of 11th Canadian Infantry Brigade assembled at Camp Borden, Ontario on 11 October 1940 and the brigade HQ was formed in December. The brigade concentrated at Camp Debart, Nova Scotia on 13 August 1941, although the Irish Regiment remained detached at Halifax. It was converted to 2nd Canadian Army Tank Brigade on 26 January 1942. The battalions of 12th Canadian Infantry Brigade concentrated at Camp Borden, Ontario in October 1940. The brigade HQ was formed in January 1941 at Camp Borden. The brigade then concentrated at Valcartier, PQ in March and April 1941. It moved to Sussex, New Brunswick in late July 1941, but its battalions were scattered through the Maritimes. It was converted to 4th Canadian Armoured Brigade on 26 January 1942.

In addition to field formations, additional units were raised in 1940 to provide internal security in Canada. For example, 13th Canadian Infantry Brigade was raised for security in the Niagara Peninsula. The battalions of this brigade were responsible for the Chippewa Power District and the Welland Canal. This brigade was sent to Naniamo, British Columbia in April 1941 to replace 10th Canadian Infantry Brigade. Two battalions were sent to Newfoundland in November 1940. Five cavalry regiments were mobilized as motorcycle regiments and were used for both internal security and support for 3rd Canadian Infantry Division's brigades in the maritimes.

The Canadian Armoured Corps was formed on 13 August 1940 and its main formation was 1st Canadian Armoured Brigade, which was established at Camp Borden, Ontario. Units for this brigade, for the most part, consisted of tank and cavalry regiments previously mobilized. In addition, tank regiments of the militia not yet mobilized became part of the Canadian Armoured Corps. This brigade became the basis for the 1st Canadian Army Tank Brigade and later the 1st Canadian Armoured Brigade.

It was decided by the Minister of National Defence in June 1940 that the Non-Permanent Active Militia would continue to have a role during the war. Recruiting for the NPAM continued through August 15, 1940. By July 1940, all NPAM infantry units not mobilized were authorized to recruit to full-strength and those that were mobilized were to form a second battalion. For example, the Cameron Highlanders of Ottawa established a 2nd Battalion on 28 June 1940 at the Drill Hall, Cartier Square, Ottawa. This battalion served in a role similar to the original NPAM unit during the war. Generally, the reserve battalions were formed at the original NPAM headquarters and served there during the war. A major change took place on 7 November 1940, when the "Military Forces of Canada" were designated "The Canadian Army". At this time all units on active duty became "Active" where all other formations were designated "Reserve." Hence the terms Canadian Active Service Force and Non-Permanent Active Militia became obsolete.