The extension will create a spur from Kennington with two new stations at Nine Elms and Battersea which will service the 25,000 new jobs and 20,000 new homes which are expected to be created in the area.
Both the project and wider Nine Elms regeneration will also provide around 22,000 construction jobs, including apprenticeships and starter roles for young Londoners.
Tunnelling is due to start in early 2017 and is expected to last six months during which hundreds of thousands of tonnes of spoil will be created.
City Hall says transporting this by road would generate than 40,000 additional lorry journeys, a volume likely to significantly add to congestion on the roads network.
Instead the spoil will be carried by barge to Goshems Farm in East Tilbury where it will be used to create arable farmland.
Mayor Johnson, said: “The whirring into action of this fantastic machine marks a key moment in the delivery of the Northern line extension.
“This gigantic conveyor belt will enable a staggering 600,000 tonnes of material to be excavated from beneath south London and transported safely by barge on the Thames.
“Once built, the extension of the Northern line will act as a major spur to regeneration and growth in the area. It will open up the Tube to thousands more Londoners, cut journey times and act as a catalyst for the creation of much-needed new homes and jobs.
“It is one of a number of major ongoing investments we’re making into improving London’s transport infrastructure, which will be ever more important as our city continues to grow.”
