Mr David Ainslie of Costerton, Midlothian died in 1900 leaving instruction to his Trustees that the residue of his estate, after a lapse of 15 years, was to be applied ‘to the purpose of erecting, endowing and maintaining a hospital or institution to be called the Astley Ainslie Institution, for the relief and behoof of the convalescents of the Royal Infirmary of Edinburgh.

A site was acquired on the south side of the city, consisting of four mansion houses and their grounds: Millbank, Southbank, Canaan House and Canaan Park. (Morelands and St Roque’s House and their grounds were added later.)

In 1923 an experimental unit of 34 beds was opened. By 1930, 120 beds were available. It was agreed that the Infirmary’s Convalescent House at Corstorphine would be used mainly for patients requiring a short convalescent period before returning to normal life. The Astley Ainslie would provide for those requiring longer care and supervision to fit them for a normal life; it might also occasionally take in infirmary patients who needed to be built up for surgery; it would not be used for patients with a chronic disability or debility due to old age, where a return to normal health would not be expected.

The Astley Ainslie grew from being a convalescent hospital to become a leading rehabilitation centre, and a leading training school for Occupational Therapists.

In 1948 it came under the South Eastern Regional Hospital Board and in 1954 it was linked under a single Board of Management with Edenhall Hospital, Musselburgh.In 1974 it became part of the South Lothian District of Lothian Health Board. By 1979 it had reached a capacity of 200 beds.

In 1986 it became part of the United Hospitals Unit.