David Gervan McMillan was born in New Plymouth on 26 February 1904, the eldest of three children of Annie Gertrude Pearce and her husband, David McMillan, who had a dairy farm near Stratford. Gervan was dux of Stratford Technical High School in 1921, and stayed on to gain a Taranaki Scholarship the following year. His report remarked that he had 'a fine head, and plenty of perseverance', combined with sporting ability; his 'influence in the school is very good'.
The scholarship helped finance McMillan's medical training at the University of Otago, from where he graduated MB, ChB in 1929. He described himself as particularly interested in social work and from 1923 he had been associated with the New Zealand Labour Party. He believed that good health required more than rendering medical assistance; this broader understanding of the social, economic and educational factors influencing health was at the heart of his views on welfare services.
While in Dunedin McMillan met Ethel Emma Black, who became history mistress at Nelson College for Girls in 1927. They were married on 4 September 1929 in St Andrew's Presbyterian Church, Gisborne. The McMillans, who were to have two sons, settled in the small northern Otago town of Kurow, where in February 1929 Gervan had become locum for William Watt. A few months later McMillan bought the practice, but wrangles over the purchase continued for several years and involved the Otago division of the British Medical Association. Although both parties aggravated the situation, McMillan's reputation within the medical profession was damaged.
Gervan McMillan's uncomfortable relationship with his peers, perhaps masking a lack of confidence, was in sharp contrast to his attentive and friendly dealings with his patients. He worked extremely hard, and became renowned for fast and furious driving around his large practice. A few miles from Kurow, workers constructing a dam on the Waitaki River were living in harsh conditions. In November 1928 the Waitaki Hydro Medical Association established a health insurance scheme, and employed McMillan as medical officer. As the number of workers increased so did his earnings. This attracted some criticism, although his generous dispensing of medicines, heavy travel costs, and the inflated price for the practice reduced his actual income.
In the early 1930s McMillan became increasingly active in politics. He saw the depression as a threat to democracy and in 1933 wrote a pamphlet on the right-wing New Zealand Legion, comparing it to the Ku Klux Klan and 'Hitlerism'. Many leading figures in the Labour Party stayed with the McMillans at Kurow, and he also associated closely with Arnold Nordmeyer, the local Presbyterian minister (who was later to join him in politics), and schoolteacher Andrew Davidson.
During this time McMillan, who strongly believed in the practical application of the teachings of Christianity, developed his ideas for a free, universal and comprehensive national health service. Adopted in principle at the Labour Party's annual conference in 1934, his plan was published as A national health service: New Zealand of to-morrow. He read widely about overseas health services, but it was his experiences at Kurow that showed how a scheme based on the concept of justice rather than charity could operate to the benefit of both doctor and patient. By 1934 the main work on the Waitaki hydro scheme was complete, the camp began to empty and McMillan's practice dwindled. At the end of the year the family moved to Dunedin, where they bought a house with a practice attached.
When Labour swept to victory in 1935, McMillan defeated William Downie Stewart in Dunedin West. He soon became a staunch supporter of John A. Lee in an increasingly divided caucus. Lee, McMillan and others believed that the party's leadership was too conservative, especially on financial issues, and sought greater use of credit and encouragement of local industry. McMillan, for example, promoted the establishment of a synthetic ammonia factory.
In its election campaign Labour had pledged to introduce a national health and superannuation scheme. In July 1936 Peter Fraser, the minister of health, appointed McMillan chairman of the parliamentary National Health Insurance Investigation Committee, which was to work out the details for a national health service that was both financially viable and acceptable to the medical profession. The committee's report, which closely followed McMillan's earlier proposals, provided the basis for the health provisions of the 1938 Social Security Act. McMillan, however, was unable to gain the support of the medical profession. He was then appointed to chair committees of inquiry into abortion (1936–37) and into maternity services (1937–38). In all three of these influential reports he emphasised the central role of the medical practitioner.
After the death of Prime Minister Michael Joseph Savage and Lee's expulsion from the party in 1940, McMillan challenged for the leadership, but was easily defeated by Fraser. He was elected to cabinet by caucus vote and appointed minister of marine and minister in charge of prisons and scientific and industrial research. His administration of the prisons portfolio was controversial. With his customary zeal, and taking the unorthodox approach of visiting prisoners, he tried to improve conditions and change attitudes, believing that prison should rehabilitate as well as punish offenders. Critics attributed a series of escapes from Mount Eden prison in 1940 to McMillan's policies.
Gervan McMillan's career in national politics was short. He resigned his portfolios in 1941 and did not contest the 1943 election. His support for Lee and failure to win the confidence of the medical profession counted against him with the party leadership, although his cool relationship with Fraser had been evident since 1935. Furthermore, McMillan had been diagnosed with severe hypertension, a condition then practically untreatable. As he had a wife and young family to support, he tried to supplement his modest parliamentary salary by seeing patients when he was in Dunedin.
In 1943 McMillan returned to Dunedin and developed a busy medical practice. He set up the first group practice with associates working in different parts of the city. Many local doctors initially did not join the social security scheme, and some accused McMillan of setting it up and then milking its benefits. He defended himself, and maintained that the scheme was intended to benefit doctors, many of whom were struggling financially.
Although McMillan had retired from national politics, he remained active in local affairs. In 1935 he had won a seat on the Labour-controlled Dunedin City Council; he served until 1941, then from 1944 to 1947, and again in 1950–51. He unsuccessfully challenged for the mayoralty in 1941 and 1944. In 1941 he was elected to the Otago Hospital Board; he was its chairman from 1944 to 1947, and was a keen proponent of the development of Wakari as Dunedin's secondary hospital. At the time of his death he was deputy mayor and chairman of the hospital board's benevolent committee, assisted by Ethel McMillan who was also a member of the board and the council. The New Zealand Medical Journal remarked that even McMillan's political opponents acknowledged his 'zeal and impartiality' during his chairmanship of the Otago Hospital Board.
Gervan McMillan died in Dunedin on 20 February 1951, shortly before his 47th birthday. He was survived by Ethel, who was Labour MP for Dunedin North from 1953 to 1975, and by their two sons.