During World War II, like most Militia units, the 11th Battalion performed garrison duties for most of the war. It was mobilised for war service at Melville, Western Australia, in December 1941. In the early part of the war, the battalion remained in Western Australia, and was assigned to the 4th Division, as part of the 13th Brigade, before later moving to the Northern Territory in 1943, where the 13th Brigade was deployed to defend Darwin, replacing the 3rd Brigade. Later in 1943, the battalion was gazetted as an AIF battalion, after more than 65 percent of its personnel volunteered to serve outside Australian territory. As the threat to Darwin passed, its garrison was reduced and the 13th Brigade was reorganised to prepare it for operations in the Pacific. The battalion, along with the rest of the 13th Brigade, was later transferred to the 5th Division and saw service against the Japanese during 1945 in the New Britain Campaign. Due to the large size of Japanese forces on New Britain, the Australian campaign was focused mainly upon containment. After landing at Jacquinot Bay the battalion was based around the Tol Plantation and conducted patrolling operations to restrict the Japanese to the Gazelle Peninsula. Little contact was made and actual combat was limited; the battalion's casualties amounted to three dead and four wounded. Following the end of the war, the 11th Battalion occupied Rabaul, before being disbanded on 11 April 1946.