On the outbreak of the First World War, Mary decided “...to convert the Cottage Hospital into a Military Hospital for the wounded. She then turned the old Riding School and Tennis Court at Woburn Abbey into another, much bigger, hospital. The old Cottage Hospital had just 20 beds, the new hospital at the Abbey initially held 80 beds, with a further 20 being added soon after. The staff for the combined hospitals included 24 fully trained nurses. For hospital surgeon, Mary was fortunate enough to secure the services of an exceptional London consultant, Mr Bryden Glendining, who was to become a lifelong friend. A total of 2,453 serving soldiers passed through the combined hospital. Yet in August 1914 her main anxiety was that her hospital would be rejected, just as her offer of the yacht had been.

"‘If they don’t send me patients, I shall ask for Germans!..’ she wrote to Zoë.

"The speed at which she moved was remarkable. On 5th August 1914 she was in Plymouth on board the Sapphire. On 11th August she spent the day in London at the War Office ensuring that the Director of the Army Medical Corps did not forget the Cottage Hospital, meeting the Head Matron of the Red Cross and arranging for University College to send those patients who were ‘fit to travel’ to Woburn. On her return to Woburn she found that she had had 150 applications to join her corps in 24 hours, with double the number expected in the next 24 hours. By 19th August, 24 beds were ready, with the hospital accepted by the War Office and properly registered. On 7th September the first soldier patients were admitted.

"Mary’s enthusiasm and drive amused as well as impressed those about her: Herbrand remarked that he felt sorry for the first patient to arrive, for there would be such jubilation over him.”