Leonard was the son of Joseph and Caroline McMillan and was a resident of Cambridge. After completing his schooling he became a carpenter and secured employment with Crowther and Bell's Livery Stables. He was a well-known sportsman and was one of the best riders in the Waikato. He was also a very good shot. Not suprisingly, when he enlisted in May 1915 he chose to join the 16th Company (Waikato) of the Auckland Regiment. After training he embarked from Wellington on the 9th of October, 1915. He sailed with the 7th Reinforcement of the Auckland Infantry Battalion, headed for Suez in Egypt on either the SS.Aparima, the SS.Navua or the SS.Warrimoo.
On transfer to France he arrived in time to serve in the Somme offensive. In late September the 1st Battalion was brought up to Gird Trench near Flers. On the 27th, the Battalion moved across no mans land. They came up agains uncut wire. Desperately they rushed through any gaps in the wire. German machine guns were trained on the gaps and there were many casualties. The Battalion pressed on and got into the German trenches and after fierce fighting the enemy was running all along the line. The objective was taken but at great cost. 800 men went over the top at the Gird system; when the line was taken, there were 200 left. The 16th Waikato company only had Lieutenant Hogg and 6 men left. The courage and tenacity of the New Zealand Division was recognised by all. On the 28th, the Battalion was withdrawn to a rest area. Leonard died on the 30th of September, 1916, aged 24. His body was never found and he is commemorated at the Caterpillar Valley Memorial, Longueval, France.