[3][1] Before the First World War he had his own legal practice at 5 George Street in the New Town and was living at "St Abbs", a villa on Russell Place in Trinity.

On 21 March 1918 near Saint-Quentin, France, when the enemy had penetrated the British line, Lieutenant Ker, with one Vickers gun, succeeded in holding up the attack, inflicting many casualties.

He then stayed at his post with a sergeant and several men who had been badly wounded, beating off bayonet attacks with revolvers, the Vickers gun having been destroyed.

Although exhausted from want of food and gas poisoning, as well as from fighting and attending to the wounded, Lieutenant Ker only surrendered when all his ammunition was spent and the position overrun - he had managed to hold 500 of the enemy off for three hours.

[8] On 11 November 1920 he was one of the 100 VC winners chosen as the guard of honour, escorting the gun carriage to the Tomb of the Unknown Soldier in Westminster Abbey.