Green Park Undergound Station is located on Picadilly and was first opened in March 1907 - slightly later than the rest of the stations on the line. At first it was served only by the Picadilly Line (at the time the Great Northern, Picadilly and Brompton Railway) but now also connects with the Victoria and Jubilee Lines. On opening the station was named Dover Street, after the side road on which the entrance was located and where lifts and an emergency staircase led to the platforms.
In the 1930s, as part of improvements to the Picadilly Line, the Dover Street lift access was replaced by escalator access from further west along Picadilly. When completed in 1932, the station was re-named Green Park with passenger entrances to the north and south of Picadilly leading to a subterranean ticket hall.
In 1938 some of the disused passages resulting from the changed access route were designated as a safe refuge for the Prime Minister although by the end of 1940 this plan was abandoned in favour of nearby (and disused) Down Street. Instead the area became asn emergency headquarters for London Transport Executives and support staff - perhaps 25 people in total. These passageways were used post-war as a mess/rest room for staff and for storage.
The Victoria and Jubilee Line platforms were opened in 1969 and 1979 respectively and there are underground connections between all three lines. In 2018 Green Park was the 13th busiest station on the London Underground with over 37 million passengers using it per annum.