Intending to study medicine at Glasgow University his plans were disrupted by the First World War and he instead enlisted in the Royal Army Medical Corps where he was posted to the 3rd Scottish General Hospital and quickly rose to Acting Sergeant.

[3] He was 21 years old, and a lieutenant in the 3rd Battalion, The Seaforth Highlanders (Ross-shire Buffs, Duke of Albany's), British Army during the Battle of Arras when the following deed took place for which he was awarded the VC.

On 11 April 1917 north of Fampoux, France, during the initial advance, Lieutenant Mackintosh was shot through the right leg, but although crippled, continued to lead his men, and captured the trench.

He then collected men of another company who had lost their leader and drove back a counter-attack, when he was again wounded and although unable to stand, nevertheless continued to control the situation.

[5] His dying moments are immortalised in Fettes College's War memorial, which features a statue of him urging his men onwards and bears the legend "Carry on".