On 6 August 1915, Mackay was involved in the Battle of Lone Pine. When the attack began, Mackay went over the top at the head of an attack. He ignored the first Turkish trench, taking a direct line to his objective. Firing from the hip, he shot and killed several Turks in the trenches below. Mackay positioned himself at the junction of two trenches, shooting down more enemy troops. When no others appeared, he came to the belief that the trenches were in Australian hands and ran across the junction into a wide bay. The first man who attempted to follow was shot dead, as were the next two after him. The rest decided not to follow. Mackay took up a position on a fire step, a raised part of the trench floor which allows men to fire over the top. Three Turkish soldiers appeared in the trench. Mackay attempted to fire but his magazine was empty. He lunged at the Turks, grazing one and making all three run. Mackay then instructed his party to fortify the position with sandbags. As the barricade was built up, it became possible for the rest of the party to join Mackay. The post became the north eastern corner of the new Australian position at Lone Pine. It was in an exposed position and came under hand grenade attack from Turkish troops. Mackay was slightly injured in one such attack. When Lieutenant Jack Massie was sent to relieve Mackay during the night, Mackay refused to leave the post.