On 23 August 1918, he was 29 years old, and a lieutenant in the 8th Battalion (Victoria), Australian Imperial Force during the First World War, when the following events occurred.
Lieutenant Joynt took charge when his company commander had been killed. When the leading battalion had been demoralized by heavy casualties, he rushed forward and reorganized the remnants of the battalion. Having discovered that heavy fire on the flanks was causing delay and casualties, he led a frontal bayonet attack on the wood, capturing it and over eighty prisoners, thus saving a critical situation. Later, at Plateau Wood, after severe hand-to-hand fighting, he turned a stubborn defence into an abject surrender.