An eye defect prevented him from becoming a surgeon.

1884 - appointed anaesthetist to Charing Cross Hospital, London.

1885 - appointed to the National (later the Royal Dental) Hospital

1886 - appointed to the London Hospital

Hewitt rapidly became known as an outstanding anaesthetist and teacher, writing many papers and delivering many lectures on improving apparatus and techniques in anaesthesia. Always concerned that students and doctors should receive proper instruction in the subject, he was foremost in persuading the General Medical Council that the study of anaesthetics should form part of the medical curriculum.

Hewitt wrote three textbooks, of which the most important, 'Anaesthetics and their Administration', was published in 1893. Appointed Anaethesthetist to Edward VII and later George V. In 1907 he was given a knighthood for services to medicine.