Johnston joined the Australian Imperial Force on 18 August 1914 as a lieutenant colonel commanding the 2nd Field Artillery Brigade. The brigade sailed for Egypt during October 1914. Guns of the 2nd Field Brigade's 4th Field Battery landed at Anzac Cove on 25 April 1915 and were the first to open fire on the enemy. The rest of the brigade, less the 6th Field Battery, which was diverted to Cape Helles, came ashore the next day, eventually settling on McCay's Hill and MacLagan's Ridge. Johnston was designated as commander of the 2nd Division Artillery but became acting commander of the 1st Division Artillery while Brigadier General Talbot Hobbs was acting 1st Division commander from August to October 1915. He then commanded the 3rd Infantry Brigade until late November.