John Godwin Banwell was born in Pegu, Burma, the son of Godwin Edward Banwell. He was educated at Merchant Taylors' School, Moor Park, and then went on to St John's College, Oxford University. He completed his clinical studies at Middlesex Hospital. From 1956 to 1959 he held house officer posts at Middlesex and Brompton hospitals. He was a senior registrar at Birmingham United Hospitals from 1963 to 1966. He subsequently moved to the USA.
At the time of his election to the Fellowship of the College, in 1979, he was based at the University of Kentucky College of Medicine. He listed his interests as reading, gardening and music. He married Bland Constance Turner in 1961. They had two sons and two daughters.