At the outbreak of the First World War in 1914, Jacob went with the Meerut division, part of the Indian corps, to the Western Front. He saw action at the closing stages of the Battle of La Bassée in October 1914. Promoted to temporary brigadier-general on 5 January 1915, he was appointed to command the Dehra Dun Brigade, and led the brigade at the Battle of Neuve Chapelle in March 1915 and the Battle of Aubers Ridge in May 1915. Promoted to temporary major-general on 7 September 1915, he became General Officer Commanding the Meerut division and led the division at the Battle of Loos in October 1915. With the Indian Corps was preparing to leave the Western front, he was appointed to take over the 21st Division of the "New Armies" on 18 November 1915. Promoted to the substantive rank of major-general on 1 January 1916 and despite being wounded in March 1916, he was promoted to temporary lieutenant-general on 28 May 1916 and appointed to command II Corps in the Fifth Army in September 1916. During the Battle of the Somme, his corps undertook the British attack during the Battle of Thiepval Ridge in September 1916 and the subsequent assault on St Pierre Divion during the Battle of the Ancre in November 1916. He remained in command of II Corps, having been promoted to the substantive rank of lieutenant-general on 3 June 1917, for the Battle of Passchendaele in Autumn 1917.