He specialised in obstetrics and was a leading authority on foetal osteomalacia. He became a Director of the Department of Obstetrics and Gynaecology at the Union Medical College in Beijing (a teaching hospital funded by the Rockefeller Foundation), President of the Chinese Society of Obstetrics and Gynaecology and worked as secretary to the medical committee of the Lord Mayor's Fund for the Relief of Distress in China. He was awarded the Army and Navy Medal by the Chinese Republic and was made a Fellow of the Royal College of Obstetricians and Gynaecologists in 1929. Maxwell returned to England at some point after 1935 (possibly as a result of the invasion of Beijing by the Japanese in 1937) and lived at Brinkley, Cambridgeshire. He was elected consultant obstetrician and gynaecologist at the nearby Newmarket General Hospital. He married Edith Lilly Isaacson in 1899 (who, as a proficient artist, illustrated some of her husband's research papers) and they had one daughter, Marjorie Gordon Maxwell (later Steen), born in 1908. John Maxwell died suddenly near his home on 25 July 1961, at the age of 89, his wife having predeceased him on 14 October 1954.