[1] He then undertook further training at St Thomas' Hospital, London, and worked as a physician in Paris and Vienna.
[1] In 1911, he earned the degree of Doctor of Medicine (MD) from Dublin and was also awarded a Diploma in Public Health (DPH) by the University of Oxford.
[1] On 17 November 1909, McCarthy was commissioned in the Royal Army Medical Corps, Special Reserve of Officers, as a lieutenant (on probation).
[10] On 30 November 1916, he was promoted to acting lieutenant colonel upon his appointment as commanding officer of a field ambulance.
This officer, hearing that an Aid Post was filled with wounded who could not be evacuated by the normal route, owing to the enemy bombardment, led a party of stretcher-bearers through heavy shell fire and reorganised the evaluation.
[19] In March 1919, he was awarded the Distinguished Service Order (DSO) for his actions in the final days of the war.
During the operations connected with the crossing of the Sambre, 4th to 8th November 1918, he exhibited marked gallantry and devotion to duty.
He was in command of the advanced dressing station and maintained the closest touch with the front line troops, arranging for the immediate evacuation of casualties as they occurred.
He worked continuously for three days and three nights, and his energy, able management and devotion to duty saved many lives.In addition to medicine, McCarthy chose to study law and was called to the bar at Inner Temple in 1920.
[27] Following the outbreak of the Second World War, he rejoined the Royal Army Medical Corps on 26 January 1940 as a lieutenant.