It was opened in 1926, when the City and South London Railway (C&SLR) was extended from Clapham Common to Morden.

[1] The City and South London Railway needed to enlarge its tunnels to accommodate larger rolling stock and the London Electric Railway obtained permission to construct tunnels to connect the C&SLR at Euston to its Hampstead Tube station at Camden Town.

[3][4] Increases in the costs of materials and labour following the war meant that building and maintaining underground railways was no longer economic, and by 1921 the country was in recession, with nearly two million workers unemployed.

[5] In order to ease the situation, the government passed the Trade Facilities Act 1921, which enabled companies involved in projects that would create employment to borrow money, with the treasury guaranteeing both the capital amounts and the interest payments.

[8][9] The Trade Facilities Act 1921 had allowed the treasury to guarantee a total of £25 million,[10] and the scheme was extended by the passing of the Trade Facilities and Loans Guarantee Act 1922, which increased the amounts of funding to £50 million, while extending the time limits by a year.

[11] With the prospect of being able to borrow more money, the London Electric Railway obtained an act that authorised an extension of the Hampstead Tube from Charing Cross (now Embankment) to Kennington in 1923, while the City and South London Railway obtained an act for an extension southwards from Clapham Common to Morden, where a new depot would be constructed.

[13] The line southwards from Clapham runs almost entirely in tunnel, emerging into the open air just to the north of Morden station.

[14] Tracks continue southwards beyond the station, passing under the A24 road to reach the depot, which is only accessible from the northern end.

At its southern end, the depot is hemmed in by Morden South station and the tracks of the Sutton Loop Line.

[21] It is unclear whether any trains of gate stock were ever stabled at the depot, as although both types of train ran on the Northern line, the earlier gate stock was only used on short workings from Golders Green and Highgate to Charing Cross, and from Euston to Tooting, but was never used in passenger service as far south as Morden.

[19] The 1935-40 New Works Programme which London Underground initiated included the provision of new trains, and delivery of 1938 Stock began in May 1938.

[23] Delivery of 1938 Stock continued into the early years of the Second World War, and by mid-1941, there were 96 seven-car trains operational on the Northern line, with 96 spare cars, so the changeover was complete.

As a result of the wartime conditions, a considerable number of new cars were stored around the system, some of which had been commissioned, but some of which could not be, because they were missing parts or motors.

Morden Depot was used to stable 39 trains to run the passenger service at that time, with Golders Green supplying 17, and the remaining 39 being stabled at Highgate and Edgware Depots, and on sidings at High Barnet, Highgate Woods, and Golders Green.