At the outbreak of the First World War in August 1914, Sinclair was called up by his unit the 1 Battalion, Lovat Scouts, Highland Mounted Brigade. In September 1915 he sailed with his battalion to Gallipoli, landing at Suvla Bay on the night of the 26 and 27of September. (See also Lance Corporal Donald Mackay, Melness.)
At the beginning of October the Lovat Scouts were moved forward into the front line trenches, two hundred yards from the Turks. The Scouts now began to attack the enemy, patrolling at night and launching sniper attacks during the day, the men from the Highlands using all their deadly stalking skills to kill their enemies.
Conditions in Gallipoli were horrendous; there was always a lack of water due to the terrible re-supply system. The whole beachhead was under constant Turkish observation, shells raining down on to the beaches day and night as the Turkish infantry launched fanatical frontal attacks on the British lines.
Private Sinclair Mackenzie was severely wounded at Suvla Bay and taken by stretcher- bearer to the field hospital. He was evacuated from the field hospital to a hospital ship, where he died from his wounds and was buried at sea on the 27 of October 1915, aged thirty- five years.