Born near Doncaster, the son of a country gentleman. Studied art in Paris, intending to become a painter, but chose to study surgery instead.
Student at St George's Hospital Medical School. House surgeon 1838, demonstrator of anatomy and the first curator of the museum at St George's Hospital, possibly in 1840[?]. Hewett set up the system for recording post mortem examinations at the hospital. Lecturer on anatomy 1845. Assistant surgeon 1848-1861, surgeon 1861-1875, consulting surgeon 1875-1891.
FRCS 1843. President of the Pathological Society of London and the Clinical Society. Arris and Gale Professor of Human Anatomy and Physiology, member of the council, chairman of the Board of Examiners in Midwifery, vice-president and president of the Royal College of Surgeons. Surgeon-extraordinary to Queen Victoria 1867, sergeant-surgeon extraordinary 1877 and sergeant-surgeon following Caesar Hawkins 1884. Surgeon to Prince of Wales, afterwards King Edward VII. Baronet 1883.
Specialisms: Anatomy, head injuries.
Married Sarah Cowell in 1849; they had two daughters and one son. Died 19 Jun 1891 at Horsham, where he had retired to. He gifted his collection of water colour paintings 'to the nation' in 1891.