Michael Herbert Watt was born at Green Island, near Dunedin, on 16 March 1887, the 11th of 13 children of Scottish-born parents Michael Watt and his wife, Isabella Shand. His father was minister of the local Presbyterian church and taught at Dunedin's Presbyterian Theological College from 1874 to 1919.
Michael Watt attended Otago Boys' High School from 1900, then in 1905 entered the University of Otago Medical School, graduating MB, ChB in 1910. Of the fourteen doctors who graduated that year, five spent at least part of their careers as hospital medical superintendents, and six served during the First World War. Watt, however, was destined to follow a very different career path. As the second recipient of the University of New Zealand's recently established travelling scholarship, he spent a year gaining clinical experience at hospitals in London, Wolverhampton and Dublin. In 1911 he returned to New Zealand and set up in private practice at Ngaruawahia. The smallpox epidemic of 1913 had a profound effect on his subsequent career, bringing him into contact with officials from the Department of Public Health and reinforcing his belief in vaccination.
On 28 August 1913, at Timaru, Watt married Mary Roberta McCahon, the daughter of a local merchant; they were to have two sons. Watt had obtained his MD the previous year, and in 1914 he accepted a demonstratorship in physiology at the Otago Medical School. The following year he transferred to a full-time post in anatomy, on the recommendation of Professor W. P. Gowland, who was concerned that Watt might be lured elsewhere. These fears were soon realised. In May 1916 Watt was recruited by the Department of Public Health as a part-time district health officer; the following April he took up a full-time post in Wellington, having become the second recipient of Otago University's diploma in public health. He was the first doctor born and trained in New Zealand to be appointed a district health officer.
Marked for high office from the outset, Watt became the first director of the Division of Public Health when the department was reformed in 1920, and was appointed deputy director general of health five years later. That year he undertook a study tour of Japan and North America, which strongly influenced his subsequent implementation of immunisation and public health nursing policies.
Watt became director general of health in 1930 at a time of retrenchment. When the economy began to improve he helped to establish the Journal of the New Zealand Branch of the Royal Sanitary Institute in 1934, hailing this as the start of a new era of progress in public health. Watt's status and diplomacy also ensured that in the late 1930s the Department of Health was rarely drawn into the dispute between the Labour government and the medical profession over health policies; the department emphatically denied any 'share in the paternity' of the 1938 Social Security Act. His third major achievement in the 1930s was to mastermind the establishment of the New Zealand Medical Research Council in 1937. In recognition of his contribution to public health, Watt was made a CBE in 1935.
Sponsored by the Rockefeller Foundation, Watt undertook an extensive study tour of North America, Britain and Scandinavia in 1938. He returned with a clear vision of the direction in which New Zealand's public health system should proceed, submitting detailed proposals in June 1939. These included the appointment of an industrial hygienist, improved collection and interpretation of vital statistics, greater emphasis on health education and dietetics, an intensified anti-tuberculosis campaign, and the creation of a national health institute. The implementation of this agenda was hindered by the outbreak of the Second World War, but most of Watt's vision was realised by the end of the 1940s.
In 1939 Watt was appointed chairman of a wartime medical advisory committee, and in 1942 he became controller of the Hospitals Emergency Precautions Service. Despite his heavy domestic schedule he also looked to expand New Zealand's role overseas. Following a meeting with Fiji's chief medical officer in 1944, Watt launched the South Pacific Board of Health, which was intended to standardise approaches to health and to promote research.
Unlike T. H. A. Valintine, his predecessor as director general, Watt was both liked and respected by his colleagues, with whom he claimed to have worked in 'complete harmony' – a view endorsed by Mary Lambie, director of the department's Division of Nursing. When Watt retired in March 1947 the New Zealand Medical Journal paid warm tribute to his impartial judgement, scientific knowledge and common sense. Thick-set, and rather sombre in appearance, he was essentially a shy and modest man who disliked public speaking.
In 1949 Michael Watt was appointed director of the Far East mission of UNICEF. A routine medical examination prior to departure, however, revealed that he was suffering from chronic lymphatic leukaemia. This prevented him taking the UNICEF position, or any other post-retirement service, and contributed to his death, in Wellington, on 7 April 1967. He was survived by his wife, Mary, and his two sons, both of whom were doctors; the elder, James, became New Zealand's first professor of paediatrics in the year of his father's death.