The 3rd Brigade, part of 1st Infantry Division, mobilised with the British Expeditionary Force on the outbreak of the First World War, and was sent to France. Landon commanded it during the Retreat from Mons, the Battle of the Marne and the Battle of the Aisne, and was promoted to Major-General in October. During the First Battle of Ypres, the divisional commander, Major General Samuel Lomax, was killed in action, and Landon took acting command. By the end of the battle in November, he himself was invalided home, and was relieved as divisional commander by Major General David Henderson. He was formally replaced in command of his brigade by Lieutenant-Colonel Richard Butler on 13 November.