The beginnings of St Mark's Hospital were in a small room at No 11 Aldersgate Street where, in 1835, Frederick Salmon opened 'The Infirmary for the Relief of the Poor afflicted with Fistula and other Diseases of the Rectum'.

Tired of the restrictions of working within the establishment, Salmon decided to found his own institution to provide treatment for those conditions which were regarded as 'the most distressing that can afflict our common nature'.

Therefore, in 1838, when the number of patients had trebled, Salmon moved to larger premises at 38 Charterhouse Square, where there were fourteen beds and more space for treating out-patients.

[4] Thirteen years later, a site in City Road was purchased from the Dyers' Company and the almshouses that occupied it were converted to a 25-bed hospital.

St Mark's was unique in not employing a physician until 1948, with the arrival of Francis Avery-Jones, "the father of British gastroenterology" and pioneer of medical treatment of peptic ulcer.

Despite the stringency of the times, the Governors purchased more land on the east side of the hospital which gave room for expansion after hostilities had ceased.

In 1926 work began on a large extension which gave the hospital a new appearance[3] and provided two new wards, as well as new Out-Patient, X-Ray, Pathology and Research Departments.

The government introduced self-governing NHS Trusts and in 1992, Sir Bernard Tomlinson's Report of the Inquiry into the London Health Service proposed radical changes to the hospital groupings then in place.

St Mark's became part of the North West London NHS trust and moved to the same site as Northwick Park Hospital in 1995.

[3] In 2021, St Mark's transferred a majority of its services to the Central Middlesex site, with Northwick Park and Ealing remaining as satellite hospitals.