A surgeon at King's College Hospital, London (1840–47), Simon was appointed first medical officer of health to the City of London (1848–55) and to the central government (1855–76). In this capacity he gradually created a state medical department to administer public health, establish state scientific research, perfect the vaccination system, and supervise the medical profession. His advocacy of public health legislation resulted in the Sanitary Act of 1866, which for the first time rendered a public health law universal, scientific, and compulsory while constituting the beginnings of industrial hygiene, and in the Public Health Act of 1875, which provided a complete sanitary code that, with consolidation and adjustments, remained in force for the next 100 years. He was knighted in 1887.