Electrification had only re-affirmed that Cannon Street was primarily a commuter station, and consequently, trade at the Cannon Street Hotel (renamed from ‘’City Terminus Hotel’’) was poor. Continental services were concentrated in Victoria, and the hotel was finally closed to customers in 1931. Thereafter, the majority of the structure was converted into offices and renamed ‘’Southern House’’, whilst large banquet rooms remained available for public meetings. During the summers of 1936 to 1939 inclusive, boat trains did in fact return to Cannon Street to free up space at Victoria, but by this time there was no hotel to serve the passengers.

Ultimately, World War II signalled the end for Cannon Street’s graceful lines, and heavy bomb damage would see the eventual removal of the station’s most imposing feature. During the night which straddled the 10th and 11th May 1941, a German air raid delivered direct hits to both the trainshed and the former City Terminus Hotel. The former lost nearly all the glazing and became an iron shell deemed beyond repair, whilst the latter was internally gutted. The Charing Cross Hotel had also received bomb damage, and both this structure and the example at Cannon Street had their upper two floors rebuilt in a plain style. Unfortunately, it was the end for the magnificent Cannon Street trainshed; the ironwork was deemed beyond economical repair, but could remain in situ until an improvement or rebuilding scheme had been finalised. However, the lack of glazing called for standard V-shaped platform canopies to be erected, to protect passengers from the elements.

In 1954, South Eastern Division suburban stations began receiving prefabricated concrete platform extensions as part of the ten-car train scheme, which aimed to tackle overcrowding. In the following year, it was announced that Cannon Street would receive similar platform lengthening as part of an extensive rebuilding exercise, which aimed to revitalise the station from its bomb-damaged state. The removal of the curved trainshed had been made definite, but the former hotel was granted a reprieve, it being seen as fit for repair. The Southern Region was to follow in the SE&CR’s Charing Cross footsteps: the curved overall roof was to be replaced with a trainshed of ridge-and-furrow design. Platform lengthening would be achieved by pushing the tracks northwards, thus cutting into the original concourse, and four of the western-most platform surfaces could also be further extended across the bridge to accommodate twelve vehicle formations. To replace the original concourse, a new circulating area was to be opened up within the bottom floor of the former City Terminus hotel. Sadly, however, all this was not to be, and Cannon Street was to become a bland, sixties-styled station. By the turn of 1956, rebuilding work was already underway, and soon British Rail reported that the former hotel building was in fact in a worse state than originally thought. It was decreed that it was only fit for demolition; this was despite the fact that the structure had been partially rebuilt in the immediate post war period. The consequence of this decision meant that the original 1955 plans were extensively altered: a ridge-and-furrow replacement roof was now ruled out, and eventually, the existing hotel site would become host to fifteen storey-high plain grey office blocks. The platforms were also to be buried underneath a large concrete slab, as per that which later appeared over the rebuilt station at Euston, and the platforms were to be permanently lit. This would produce a car park for the proposed office development.

On 5th April 1957, to intensify the ordeal of rebuilding work, the 1926-opened signal box beside the western tower of the station, caught fire. An electronic malfunction in the cabin’s bottom floor relay room was cited as the probable cause, and the 140-lever power frame was destroyed as a consequence. A replacement ‘’power box’’ was scheduled to come into use on the south side of the river in December of the same year, but a solution was required for the interim. The fire had been contained to the extent that half of the original structure could be salvaged and rebuilt into a smaller cabin, and this could be equipped with part of a spare power frame which the Midland Region was able to offer. The Midland Region’s equipment comprised 227 levers, of which only about a fifth were required. However, the remaining sections of the frame were by no means surplus to requirements, and these were later installed in the aforementioned ‘’power box’’ south of the river. The rebuilt timber cabin was brought into use on 6th May 1957, but was operational only until December of that year; on 16th of that month, Cannon Street Power Box assumed its functions. With reference to the latter, this was built to the same design as those brick and glass power box structures which appeared during the Kent Coast Electrification period, and was positioned on the former site of the engine shed turntable.

In Spring 1958, dismantling of the trainshed roof began in earnest, and by the beginning of the following year, it had all but gone. During 1963, the former hotel was razed to the ground, and by the end of 1965, a fifteen-storey office block abomination had risen up on the site. The concourse had, in the meantime, become a concrete wind tunnel by the addition of a large-span slab roof over the top of it, as per the rebuilt Euston. However, this was never extended fully over the platforms, and V-shaped asbestos-roofed canopies were retained beyond this. The rebuilding had produced a much reduced concourse area, and to conserve the remaining circulating space, toilet facilities and a parcels room had been installed at a sub-surface level. The remodelling work had also seen the demolition of the northern half of the huge side walls, in connection with the installation of a new bridge span underneath the station to accommodate a widened Upper Thames Street. It had, however, been decided to retain the iconic towers and sections of the southern retaining walls. In the meantime, the station had witnessed its last regular steam-hauled service depart on Saturday 13th June 1959. The full electric timetable for the Kent Coast Electrification’s ‘’Phase 1’’ did not come into use until 15th of that month, but Cannon Street was closed to all services on Sundays.