The building was shared with the 1st Battalion, London Regiment. Annual training for 1st London Division had just started when war was declared on 4 August 1914, and the City of London Brigade promptly mustered at Bloomsbury for mobilisation. The infantry of the division were soon posted away to relieve Regular Army garrisons in the Mediterranean or to supplement the British Expeditionary Force on the Western Front. By January 1915, only the artillery and other support elements of the division remained in England, and these were attached to the Second Line TF division (58th (2/1st London) Division) that was being formed. The City of London Brigade became 1/I City of London Bde and formed 2/I City of London Bde, which served with the 58th Division throughout the war. Colonel J. Stollery, who commanded I City of London Brigade, had been with the unit since 1874 and was too old for overseas service, so he remained behind to train the 2/I Brigade. So many recruits came forward, including many who had previously served with the brigade and whose training could be quickly refreshed, that the 2/I Brigade was soon completed, and a 3rd Line Brigade was also formed to supply reinforcements to the other two. The brigade left Handel Street on 23 August 1914 and underwent training in various locations. During the winter of 1914–15, it spent five months guarding the Northumbrian Coast. In August 1915 the 36th (Ulster) Division was being readied for service. Its infantry were largely drawn from the Ulster Volunteers and had already received weapons training before the war; the artillery however were newly raised Londoners, and the drivers were still being taught to mount and dismount from wooden horses. The 1st London Divisional Artillery were therefore attached to the Ulster Division until its own gunners were ready for active service. In September 1915 the 1/I City of London Bde moved to Bordon to re-equip with modern guns and prepare for overseas service. It then accompanied the Ulster division to France, landing at Le Havre on 4 October 1915. 1/I Bde went into the line on 9 October, and first went into action at Colincamps. In December, the Ulster Division's artillery arrived from England, and the London Divisional Artillery was transferred to the 38th (Welsh) Division, which had also arrived in France minus its own artillery. 1/I City of London Bde served with the Welsh Division from 11 December 1915 to 1 February 1916.