John McLean was born at Swan Hill, Victoria to Angus and Jane McLean. He was educated at Ballarat High School before entering Melbourne University where he graduated in 1926. His appointment to the Alfred Hospital in Melbourne in the same year was the beginning of his life long association with it until his death and, indirectly, to the birth of haematology as a discipline in Australia. In that year the Baker Medical Research Institute was created at the Hospital and Dr John Fiddes, a physiologist from Aberdeen, was appointed as the full time pathologist to develop experimental pathology. John McLean was appointed in 1927 to be Dr Fiddes' assistant and clearly came under his influence. Within three years he had completed and published a number of original works on haematology and been awarded his doctorate degree of medicine for his thesis on the haematological changes caused by experimental lead poisoning.

After spending five years furthering his training in clinical medicine in England, he returned to Melbourne and in 1936 he established a private clinical practice, returned to the Alfred Hospital as a physician and to the Baker Institute as a haematologist. There he continued his researches and published a succession of original papers including one on the assessment of the use of sternal puncture. He also became interested in blood transfusion and developed a rotary roller pump for transferring blood directly from a donor to the patient; his pump was improved by the surgeon Julian Smith after whom it became known.

In 1947 he was appointed honorary physician to the newly created haematological clinic and retained this appointment until the end of 1962 when he reached the hospital retiring age. During these seventeen years he constantly sought better diagnostic methods, developed new ways of using direct transfusion techniques (described by him in the Australian Medical Association's Monograph No 5) and used the newest isotopic and chemotherapeutic drugs for haematological malignancies. His knowledge and experience as a haematologist were in constant demand. Further he fostered the interest of others in haematology in many ways.

He was a foundation member of the Haematology Society of Australia, its president from 1963 to 1965 and a vice president of the Asian and Pacific division of the International Society of Haematology. Following his retirement from the Alfred Hospital he was appointed honorary consultant haematologist and oncologist, a life long appointment in recognition of his singular achievements. Although he never claimed to be the father of Australian haematology, there is little doubt he was.