After the war Donald continued his studies at Birmingham University Medical School from where he graduated in 1924. The London Gazette reported that in the RAF Medical Branch he was
granted a short service commission as a Flying Officer with effect from, and with seniority of, 24th Sept. 1924. He had qualified MB, ChB in 1926; he got a DPH while in the RAF and studied Tropical Medicine but resigned his commission to look after his ailing mother. The London Gazette reported that on 24 September 1926, he transferred to the RAF Medical Branch Reserve,
Class D.2 he was promoted Flight Lt. and on 24 September 1930 was
transferred from Class D (ii) to Class D (i). After his mother's suicide Donald moved into psychiatry and got his DPM.
Donald went to work at Caterham (later St Lawrence's) Hospital, Surrey. It was here that he met his wife, Isabella MacAlister; they married in Caterham in 1936 [Surrey SE 1936, 2nd qt] with what the local newspaper reported as being a "pretty wedding".
Isabella had obtained an MA from Glasgow University in 1928 and was occupational supervisor at Caterham. She and Donald had a mutual interest in trying to realise the full potential of the mentally handicapped; Donald enjoyed the challenge of looking after the medical well-being of patients whose problems with communication often made it difficult for them to describe their symptoms. He also encouraged physical activities, sports including cross-country running and an annual sports day. He was at one time Second Assistant Medical Officer at Shenley Hospital, Hertfordshire, and Certificate Officer for Mental Deficiency for the Lancashire Mental Hospital Board. He subsequently became Deputy-Superintendent of Botleys Park Hospital for the Mentally Handicapped, Chertsey, Surrey, and by 1940 was living at The Croft, Botleys Park.
Donald had remained active in the reserve forces and by 1939 he was dedicating much of his spare time to giving medical examinations to potential RAF pilots. When huts were erected creating a war hospital adjacent to the mental hospital, officials were amazed to discover that the latter was already geared up to receive war-wounded.