Augusta Crichton Stuart, 4th Marchioness of Bute, offered Mount Stuart to the Admiralty as a naval hospital at the outbreak of World War One. Far enough away from the main theatre of war but close to the clearing hospital in Edinburgh and near to Glasgow, the Isle of Bute was in the perfect position for a hospital of this kind. Lady Bute oversaw the transformation of the house and organised the provision of wards, x-ray facilities and an operating theatre.
Mount Stuart was run as a hospital from 1914 to January 1919 under the direction of William McEwen (1848-1924) who was a Bute native and pioneer in brain surgery.
During this time the Marchioness assisted with the treatment of over 2,000 patients, after completing nurses training at the Scottish General Hospital at Stobhill. She was made a Dame of the British Empire by King George V in recognition of her work during the War.
Over the course of its life as a hospital Mount Stuart saw many soldiers pass through its wards with a variety of injuries and diseases such as influenza, malaria and syphilis, TB, pneumonia, epilepsy, neurasthenia (related to shellshock), and bronchial issues in soldiers gassed in the trenches. There were 834 operations carried out under general anaesthetic at Mount Stuart including 48 appendectomies, 14 amputations, 28 bones plated together and the removal of 2 testicles.