It was originally raised in February 1916 for service during World War I as part of First Australian Imperial Force, with personnel being drawn mainly from the state of Victoria.

Making up part of the 10th Brigade, it was attached to the 3rd Division and served on the Western Front in France and Belgium before being disbanded in March 1919.

Subsequently, between July and August of that year the unit was heavily engaged in the defence of Port Moresby, fighting along the Kokoda Track.

Such was their involvement in the battle that by the time they were withdrawn they could only muster 32 men and following its return to Australia, the unit was disbanded in early July 1943.

[5] Under the command of Lieutenant Colonel Robert Rankine,[6] the battalion was raised as part of an expansion of the First Australian Imperial Force (1st AIF) that took place at the conclusion of the Gallipoli Campaign.

[5] They remained at the front for the next week, as part of their introduction to trench warfare, during which time the battalion repelled a small German raid and sent out patrols into "no man's land".

[9] After having endured a long winter in Flanders serving in mainly a defensive role, the battalion's first major engagement came at Messines, in Belgium in early June 1917.

Despite this, the 39th was quickly reorganised into a single wave,[11] and attacking on the 10th Brigade's right,[12] it subsequently overcame the initial German opposition facing them and then, during the second phase of the battle advanced south of Douve, on the southern edge of the Messines Ridge.

[15] A couple of days later, the battalion was recommitted to the fighting and ordered to advance towards Passchendaele Ridge, attacking on the morning of 12 October despite heavy rain the previous evening.

In the heavy fighting that followed, the battalion managed to secure its first objective, but was forced to withdraw when its flanks became threatened as neighbouring units had been unable to advance with them through the thick mud.

[17] In the spring of 1918, when the German Army launched its last effort at victory, known as the Spring Offensive, the 39th was among the many Australian battalions that were hurriedly moved south to France in order to stem the tide of the German onslaught towards Amiens;[5] fighting a series of defensive actions in the Somme between late March and early June.

[5] The battalion undertook its last major action of the war at the end of September 1918 when, serving alongside the Americans, they breached parts of the Hindenburg Line along the St Quentin Canal.

[21] At this point, the battalion's remaining personnel were amalgamated with other units to form the 10th Demobilisation Regiment, with the last of its soldiers returning to Australia in May.

[5] Members of the battalion received the following decorations: two Distinguished Service Orders (DSOs), one Member of the Order of the British Empire (MBE), 14 Distinguished Conduct Medals (DCMs), 14 Military Crosses (MCs), 78 Military Medals (MMs) with three Bars, and 22 Mentions in Despatches (MIDs).

[39] The battalion headquarters was opened at Darley Camp, near Bacchus Marsh,[40] and by 8 October 1941, a nucleus of officers and senior non-commissioned officers (NCOs), many of whom had experience from World War I, had prepared the battalion for the arrival of the soldiers or other ranks (ORs) that would bring it up to its required establishment.

[46] Two days after the Japanese attack on the US fleet at Pearl Harbor and the British in Malaya, on 9 December 1941 the battalion was ordered to "prepare to move" as it was mobilised for war service.

[48] Initially upon their arrival in New Guinea in January 1942 the 39th Battalion was used to defend the airfield at Seven Mile Aerodrome near Port Moresby and to carry out various other garrison tasks such as building defences and unloading stores at the wharf.

[24][50] The Australians were forced to withdraw, however, when hundreds of Japanese marines began crossing the river under a barrage of mortar and machine gun fire.

[51] It became clear that Kokoda was lost and the following morning, under the cover of a dense mist, with the PIB's commanding officer, Major William Watson, assuming temporary command, the survivors abandoned the position and fell back towards the village of Deniki, a mile or so back along the Kokoda Track towards Isurava.

[24] At this point the fighting ceased for almost two weeks and during this time the 39th was joined by the 53rd and the 30th Brigade headquarters;[24] and Lieutenant Colonel Ralph Honner arrived from Ilolo to take command.

[54] On 23 August Brigadier Arnold Potts took over command of Maroubra Force[55] and further reinforcements arrived as first the 2/14th, 2/16th and later the 2/27th Battalions from the 7th Division's 21st Brigade also reached the area.

[24] Despite this, the situation remained bleak as the supply issue was becoming a serious problem for the Australians and the reinforcements that had arrived were also in a state of disarray having been committed to the battle in a piecemeal fashion and suffering badly from hunger and disease.

[24] Following the 39th Battalion's withdrawal from the line in September 1942, they spent a month at Koitaki before being sent back to Port Moresby in mid-October, where they were detailed to prepare defensive positions.

[24][59] During this time the 39th suffered heavy casualties, but the fighting continued and having captured the Gona Mission, the battalion moved to the Sanananda Track on 21 December, taking up a forward position at Huggins' Road Block.

[70][Note 3] Illness and disease also took a heavy toll and as a result, after six months of combat the 39th Battalion's muster roll was only seven officers and 25 other ranks.