In 1846, St Mary Abbots purchased a new workhouse site at the east of Wright's Lane (now Marloes Road) from Mr Gunter. In May 1847, after inspecting 35 other recently built Metropolitan workhouse, the Board of Guardians advertised for plans for a new building to cost no more than £9,000. The winning design, by Thomas Allom, was a Jacobean style red-brick construction.
The building was:
"to contain upwards of four hundred paupers, and has an infirmary with airing grounds detached from the main building. The first tender as accepted by the Board was 10,600 and some odd pounds; the second, with additions, amounted to 11,020l. The total length of frontage is 262 feet, the whole of which is given to the aged and infirm, with arcades for exercise and a garden in front. The able-bodied and younger classes are kept more immediately under the eye of the master and matron."
In 1868, the recently formed Metropolitan Asylums Board set up six new Sick Asylum Districts for the purposes of providing hospital care for the poor on separate sites from workhouses. One of the new Districts, named Kensington, comprised the parishes of St Margaret and St John, Westminster, together with St Mary Abbots. However, the large new hospital required by the new scheme was felt to be too expensive and the Kensington Sick Asylum District was dissolved. Instead, St Margaret and St John joined St George, Hanover Square to form the new St George's Union. St Mary Abbots then reverted to being a Poor Law Parish.