On 1 May 1945, Mackey's battalion, as part of 26th Brigade Group, landed at Lingkas Beach on Tarakan Island, off North Borneo. The island's airfield was to be captured to allow its use in operations against Borneo. Advancing inland along the Aman River, the battalion were held up by Japanese defending a stronghold known as Helen. On 12 May 1945, Mackey's company was to continue an attack that had begun three days previously and it was during this action that he earned the Victoria Cross (VC). The citation for his VC read:
Corporal Mackey was in charge of a section of the 2/3rd Australian Pioneer Battalion in the attack on the feature known as Helen, east of Tarakan town. Led by Corporal Mackey the section moved along a narrow spur with scarcely width for more than one man when it came under fire from three well-sited positions near the top of a very steep, razor-backed ridge. The ground fell away almost sheer on each side of the track making it almost impossible to move to a flank so Corporal Mackey led his men forward. He charged the first Light Machine-Gun position but slipped and after wrestling with one enemy, bayoneted him, and charged straight on to the Heavy Machine-Gun which was firing from a bunker position six yards to his right. He rushed this post and killed the crew with grenades. He then jumped back and changing his rifle for a sub-machine-gun he attacked further up the steep slope another Light Machine-Gun position which was firing on his platoon. Whilst charging, he fired his gun and reached with a few feet of the enemy position when he was killed by Light Machine-Gun fire but not before he had killed two more enemy. By his exceptional bravery and complete disregard for his own life, Corporal Mackey was largely responsible for the killing of seven Japanese and the elimination of two machine-gun posts, which enabled his platoon to gain its objective, from which the Company continued to engage the enemy. His fearless action and outstanding courage were an inspiration to the whole battalion.