John Tansey was born in Sydney, one of six children of John, a grocer in Crown Street, Surry Hills, and Anne. He won a scholarship from the Marist Brothers' School at St Benedict's Broadway to St Joseph's College, where his academic work was marked by industry and success. He played in the premiership-winning first fifteen for three years.

He entered the faculty of medicine in 1907 and graduated among the leaders of his year in 1911. After two years as an RMO at Sydney Hospital, and periods at the Naval College Works at Jervis Bay, and in the Education Department, he started practice in Crown Street. In 1926, armed with extensive experience of general practice, he was admitted MRCP London. He impressed examiners and fellow postgraduate students with his knowledge. He also studied in Vienna and New York, and returned home to start practice as a consultant physician. He had already been appointed honorary assistant physician to St Vincent's Hospital in 1919, and in 1924 was appointed to the senior staff one year after the establishment of the clinical school there. In 1929 he was appointed honorary physician to the Mater Misericordiae Hospital, North Sydney. He held these appointments until his death.

He was twice married, first to Irene Denholm-Lawrence who died in 1928, and then to Mary Lane-Mullins who was a young trained nurse at St Vincent's Hospital. There were four children of the first marriage and one son of the second. His second son Lawrence, a doctor, was killed in an aircraft on his way to active service in New Guinea in 1943.

He is remembered as good looking, tall, grey haired and dignified. Wearing horn-rimmed glasses and often with a somewhat quizzical smile, he would proceed quietly on his hospital rounds. In the days before specialisation he had particular interests in rheumatic heart disease and nephritis. In teaching he often used appropriate epigrams and metaphors. It was no trouble for him to spend some time on one physical sign, needing to be satisfied that his students fully understood its place in diagnosis. To them he appeared somewhat reserved, and he was probably shy, but in the hospital staff room he relaxed with his close friends. It was always refreshing and stimulating to join him in discussion. He would often take an attitude to provoke a good humoured argument. His approach to patients was kind and sympathetic, and to staff courteous. With his academic knowledge and careful logical approach to clinical problems he was in demand as a consultant and as a personal physician.

In 1938 he had a cardiac infarct. He had never enjoyed giving formal lectures, and after this illness there were fewer teaching rounds. However, keen undergraduate and postgraduate students continued to benefit from his wisdom during the War, and until his final cardiac illness began in 1946. He endured this with Christian resignation. John's life away from work was devoted to Mary and to their five children. He continued to read widely, and his only regular outside interest was his enjoyment of watching Rugby, particularly when his sons were playing. His friends observed that this was often too much an emotional strain as he was a surprisingly vocal partisan.