Born at Kyneton, Victoria, Alec Baldwin graduated in medicine at the University of Melbourne in 1917, and immediately joined the Australian Imperial Force. He served as a captain in France and Belgium.

After returning to Australia at the end of the war he took the DPH (Melbourne) in 1920, worked on hookworm eradication, and in 1922, obtained an International Health Board fellowship to study epidemiology, immunology and statistics at Johns Hopkins Hospital in Baltimore, and tropical diseases at the London School of Hygiene and Tropical Medicine, where he obtained the DTM&H with distinction and second place.

Then began a long association with the Commonwealth Health Department, first at Rabaul, and from 1924, as lecturer and acting director of the Australian Institute of Tropical Medicine in Townsville. An extended visit to South-East Asia in 1927 to 1928, gave him first-hand experience of many tropical diseases not seen near Australia. In later years, he conducted a number of surveys of hookworm, malaria, filariasis and more in Norfolk Island, Western Australia, Northern Territory and other areas.

In 1930, the Townsville Institute merged with the newly formed School of Public Health and Tropical Medicine in Sydney, and Baldwin took up the post of deputy director of the School (Tropical Medicine) in February 1930. In 1947, he was appointed to the newly established chair of tropical medicine within the School. He held this post until his retirement in December 1956, his successor in 1957, being RH Black.

In the Second World War, Baldwin was initially appointed to the Army as a specialist, but was soon sought by the Royal Australian Air Force, and made Director of Tropical Medicine and Hygiene with the initial rank of wing-commander, later group captain. He had the task of advising on measures to combat tropical diseases in the Air Force. He set up efficient hygiene and tropical units to minimise the incidence of these diseases among Air Force personnel, he arranged training in tropical medicine for medical officers and others, and his services were highly regarded by his seniors. He also was a member of the Services Joint Advisory Medical Committee, a small group of top experts who advised General Macarthur and General Blamey on tropical diseases, and thus played an important role in the defeat of the Japanese forces.

Quiet, retiring and modest, he did not seek publicity but was favourably known to many. In 1927, he married Beatrix Fitzmaurice and they had a son. In his retirement he lived at Epping, where he died a few days after his 80th birthday.