The 1st Light Horse Brigade was a mounted infantry brigade of the Australian Imperial Force (AIF), which served in the Middle Eastern theatre of World War I. The brigade was initially formed as a part-time militia formation in the early 1900s in New South Wales and then later in Queensland. In 1914, the brigade was re-constituted as part of the AIF and was sent to Egypt. Forming part of the New Zealand and Australian Division, during the Gallipoli Campaign it served in a dismounted role between May and December 1915. After being withdrawn to Egypt it served in the Anzac Mounted Division from March 1916 as part of the Egyptian Expeditionary Force, taking part in the Sinai and Palestine Campaign until the end of the war. It was disbanded in 1919. After the war, the AIF light horse regiments were demobilised and disbanded; however, the brigade briefly existed as a part-time militia formation in Queensland until 1921 when its regiments were reorganised into cavalry brigades.

The 1st Light Horse Brigade was initially raised as part of the Citizens Forces in the early 1900s, being formed sometime between 1902 and 1905. That formation was raised in New South Wales, and consisted of three light horse regiments – the 1st (New South Wales Lancers), 2nd (New South Wales Mounted Rifles) and 3rd (Australian Horse). The 1st had depots around Parramatta and other smaller centres; the 2nd was based in Sydney and was spread out across depots from Camden to Dubbo; and the 3rd was based in several small towns including Bega, Goulburn and Bungendore. In 1912, an Army wide reorganisation resulted in the brigade's designation being reused for a Queensland-based formation, consisting of the 2nd Light Horse (Queensland Mounted Infantry), the 3rd Light Horse (Darling Downs Regiment), and 4th Light Horse (Northern River Lancers). The 2nd had depots in Brisbane and its surrounds, spread out to Laidley and Ipswich, while the 3rd was based around Toowoomba, Warwick, Roma and other smaller centres; the 4th was based around Lismore, Casino, Grafton and a couple of other smaller centres. The previously assigned New South Wales-based regiments were redesignated and reassigned to 2nd and 3rd Light Horse Brigades.

At the outbreak of the war August 1914 the Australian Government decided to raise the all- volunteer Australian Imperial Force (AIF) consisting of 20,000 troops comprising an infantry division and a light horse brigade of three regiments to be used at the discretion of Britain. These regiments were raised from volunteers for overseas service, as the provisions of the Defence Act did not allow conscripts to be deployed overseas. Nevertheless, many of the recruits were drawn from the various militia light horse formations created as a consequence of the Kitchener Report 1910 and the introduction of Universal Training, although they were assigned to freshly raised units that were separate to the light horse regiments raised as part of the militia. Initial enlistments outstripped expectations and, as a result, a total of three light horse brigades as well as two divisional cavalry regiments were formed in the early part of the war.

The 1st Light Horse Brigade was the first light horse formation raised at the start of the war. It was part of the 1st Contingent that was hastily put together in the middle of August 1914, and consisted of 1,560 men organised into three regiments – the 1st, 2nd and 3rd – each of approximately 520 men, under the overall command of Brigadier General Harry Chauvel. The brigade's personnel were drawn from the states of New South Wales, Queensland, South Australia, and Tasmania.

Following its formation, the brigade began embarking for Egypt from Sydney, Brisbane, Adelaide and Hobart, in September and October 1914. Upon arrival in December 1914, the brigade moved into camp at Mena, where training was undertaken, and further reinforcements were received from Australia. In May 1915, the brigade was deployed to Gallipoli as reinforcements for the infantry that had landed in April, but had become pinned around a small perimeter around a beachhead at Anzac Cove; deployed in a dismounted role, the brigade was assigned to the New Zealand and Australian Division. About a quarter of the strength of each light horse regiment remained in Egypt with their horses; however, additional reinforcements were provided prior to their arrival, to bring them up to strength.

Upon arrival, Chauvel's brigade was assigned to hold part of the defensive line that the Allies had established around Anzac Cove. During this time, they undertook patrolling operations, occupied outposts, carried out sniping and worked to dig trenches and lay down wire. For the majority of the campaign the brigade's regiments maintained a defensive posture; however, in August the Allies attempted to break the deadlock on the Gallipoli peninsula, launching the August Offensive in an effort to secure the heights around Sari Bair, Chunuk Bair and Baby 700. During this effort, the 1st Light Horse Regiment carried out a feint attack at Pope's Hill, in support of the attack at the Nek. Out of 200 men committed, less than 40 survived. Meanwhile, the 2nd Light Horse Regiment undertook its own attack around Quinn's Post; this too met little success and was called off after the first wave of attackers suffered many casualties. Meanwhile, the 3rd Light Horse Regiment was held back in reserve. Following the failed offensive, the brigade occupied positions around the top of Monash Valley, which remained strongly contested. As winter approached, in early September, they were moved to a more secure location on the seaward side, to the north of Anzac Cove, towards Suvla Bay; this position included outposts at Destroyer Hill and Sazli Dere, which had been captured in late August during the Battle of Hill 60. By mid-December all three regiments were withdrawn from the peninsula, as part of the general withdrawal that followed the decision to abandon the position. The 3rd Light Horse Regiment was the first to depart, leaving on 14 December; it was followed by the 2nd on 18 December, while the 1st remained until 21 December, which was the last day. They were subsequently returned to Egypt, sailing via Mudros.

After the evacuation from Gallipoli, the Australian and New Zealand forces in the Middle East were reorganised. There were a large number of reinforcements that had arrived in Egypt at this time, and while the infantry was to be deployed to the Western Front, the mounted units were to remain in the Middle East. This resulted in the establishment of the Anzac Mounted Division, which consisted of the 1st, 2nd and 3rd Light Horse Brigades, and the New Zealand Mounted Rifles Brigade. Chauvel had been promoted to command this division, while Brigadier General Charles Frederick Cox, who had taken over command of the brigade in early November 1915, remained in command of the 1st Light Horse Brigade. He would remain in command of the brigade until after the end of the war. At this time, the brigade was provided with a British Territorial horsed artillery battery, the Leicestershire Battery, which was detached from III Brigade, Royal Horse Artillery (T.F.). Until July 1916, the brigade was supported by a machine gun section, but this was then expanded to a full squadron, equipped with 12 machine guns. A light horse training regiment was also established for each brigade, to provide trained reinforcements, while other supporting elements including signals, logistic, engineer, medical and veterinary support units were also assigned.

In the early part of 1916, the 1st Light Horse Brigade was sent to guard parts of the Nile and Suez Canal from Senussi tribesmen. This lasted from January to May 1916; afterwards it was reassigned to defend the Suez Canal. In August 1916, the brigade helped repel an Ottoman advance on the canal during the Battle of Romani, which took place 35 kilometres (22 mi) to the east; they were involved in the initial fighting as the Ottoman forces clashed with the brigade on the night of 3/4 August, which had occupied outposts throughout the area that the Ottoman commanders had detailed as the forming up position for their attack on the Suez Canal. Threatened with being outflanked, the brigade was pushed back in the early morning, but they were reinforced just in time by the 2nd Light Horse Brigade, which appeared on the brigade's right and helped restore the situation. They were followed by New Zealand and British troops who launched a counterattack later in the day, while the two light horse brigades remained in the centre. Initially too exhausted to counterattack, they were committed early on 5 August, advancing with bayonets drawn. As the tide of the fighting turned, in the following days, the brigade was committed to the pursuit that followed as the Ottoman forces began to withdraw.