During the First World War, IMechE headquarters were taken over by the Office of Works, and the National Relief Fund. The IMechE moved into 11 Great George Street as tenants of Armstrong, Whitworth & Company.
After the war, various schemes were considered for remodelling the building. In 1933, James Miller was asked to undertake the remodelling. In the Lecture Theatre, all natural light sources were removed in favour of electric lighting. The three original windows were blocked up and the walls panelled with Austrian oak wainscoting. The ceiling dome was removed and the library expanded to fill the space. 220 tip-up seats were installed.
At the outbreak of the Second World War, the Institution moved out of London to occupy The Meadows at Bletchworth in Surrey. Here, 'the bulk of the Institution's work (Accounts, Records, Membership, Examinations, National Certificates, National Service and general correspondence' was being carried out...with about twenty-eight members of the Staff (fourteen sleeping on the premises).' In July 1940, The Meadows was requisitioned by the military, and the IMechE moved back to Storey's Gate, by then a much altered building. The entrance and roof was sandbagged, and the windows netted. The basement had been turned into day and night air raid shelters by the local council.
During this time, IMechE headquarters acted as a meeting place not just for the Institution, but for other bodies including the Royal Netherland Institution of Engineers, the Association of Polish Engineers in Great Britain and the Society of Engineers and Technicians of the Fighting French Forces. Government departments also made use of the building, and in January 1943, a group of senior REME (Royal Electrical and Mechanical Engineers) used the building to plan the mechanical engineering support for Operation Overlord and the invasion of Normandy.