Although there must be many hospitals named St Mary's Hospital, the most famous is probably located in Paddington, West London. Under the 20th century the hospital had its own medical school, which later merged with Imperial College. The hospital is under the St Mary's NHS trust.

During the redevelopment of the Paddington Basin, the hospital is expected to merge with the Royal Brompton & Harefield NHS trust and move to new premises on the basin.

Famous researchers at St Mary's include Alexander Fleming, who identified penicillin, Flemings boss, Almroth Wright who advanced vaccination through the use of autogenous vaccines and Augustus Waller , whose research led to the invention of the electrocardiogram (ECG).