Initially commanded by Brigadier Leslie Morshead, it served in the United Kingdom in 1940–1941, where it helped bolster the British garrison in anticipation of a possible German invasion following the Fall of France.

The brigade's constituent units were spread across various locations in Victoria including Williamstown, Geelong, Ballarat, Warrnambool, Harsham and Stawell.

[5] However, while they were at sea they were diverted to the United Kingdom in order to help defend the island against a possible invasion by German forces following the Fall of France.

[7] Later, when the threat of invasion of the United Kingdom decreased, the decision was made to transfer the Australian forces there to the Middle East.

[11] Although the fortress was held by about 1,500 Italians supported by artillery, the Australian force dispatched only consisted of a reinforced battalion due to supply difficulties.

[13] A heavy sandstorm obscured visibility, which hindered the supporting artillery and resulted in a number of casualties when one company moved beyond the barrage.

[14] Plans were made for the 18th Brigade to deploy to Greece, but a German offensive in Libya resulted in the 7th Division being retained in North Africa.

[16] Following the armistice the division was allocated the task of undertaking garrison duties and the 18th Brigade was subsequently stationed at Aleppo, near the Turkish border to guard against a potential German attack through Turkey.

[18] Following defensive duties and training in Queensland, the 18th Brigade, under the command of Brigadier George Wootten, took part in the Battle of Milne Bay.

[21] Between 27 August and 7 September the brigade played an important role in the fighting which resulted in the first major defeat of the Japanese on land since the war had begun in December 1941.

[22] One of the brigade's soldiers, Corporal John French, of the 2/9th Battalion, was awarded a posthumous Victoria Cross for his actions during the fighting on 4 September 1942.

[37] Following the conclusion of the fighting around Shaggy Ridge in February, the brigade remained in New Guinea until May 1944 when they were withdrawn back to Australia for re-organisation.

[39] The brigade's final involvement in the war came when they landed at Balikpapan in Borneo on 1 July 1945,[7][40] as part of efforts to secure the port facilities and oilfields located there.

[39] Following the initial landing, the 18th Brigade, supported by 'D' Company, 2/1st Machine Gun Battalion,[41] secured the high ground around Klandasan.