The London Underground station is a sub-surface station, situated immediately below the mainline station. It is served by the District and Circle lines. Entrances are located on Cannon Street, Dowgate Hill, and on the mainline concourse upstairs at the National Rail station, providing an interconnection for commuters. A station here was part of the unrealised phase two expansion of the Fleet line.
The Underground station is open Mondays to Saturdays only as it is used primarily by commuters who work in the financial services industries of the City. Alternatively, passengers can walk to Mansion House, the next stop along from Cannon Street, or to Bank for the Northern, Central or Waterloo & City lines, and the Docklands Light Railway.
By 1876, the Metropolitan Railway (MR) and Metropolitan District Railway (MDR) had constructed the majority of the Inner Circle (now the Circle line), reaching Aldgate and Mansion House respectively. The companies were in dispute over the completion of the route as the MDR was struggling financially and the MR was concerned that completion would affect its revenues through increased competition from the MDR in the City area. City financiers keen to see the line completed, established the Metropolitan Inner Circle Completion Railway in 1874 to link Mansion House to Aldgate. Forced into action, the MR bought-out the company and it and the MDR began construction of the final section of the Inner Circle in 1879.
On 6 October 1884, the final section of the Inner Circle was opened along with Cannon Street station. Initially the station was served by trains from both companies as part of circular Inner Circle service but various operational patterns have been used during the station's life. The Inner Circle service achieved a separate identity as the Circle line in 1949 although its trains were still provided by the District or Metropolitan lines.
The station was reconstructed at the same time as the main line station.
London Buses routes 15, 17, 344, 521, Heritage Route 15 and night routes N15 and N47 serve the station.