It starts on the A1261 in Limehouse (although it originally extended further north to start on the A13) and heads south along the Thames. It meets the A1203 (from the Limehouse Link Tunnel at traffic lights and then goes through a tunnel of its own, at Westferry Circus. There are actually two roundabouts here, one above the other, to provide access to different levels of Canary Wharf. Although the upper roundabout is in the open air, maps claim that the lower roundabout, in the tunnel, is the one used by the A1206.
In 1922 the road ended at the entrance to Millwall Dock (departure point for the ferry to Rotherhithe) and morphed into the B124. However, it has now taken on the whole of that road and therefore continues along the river around the loop of the Isle of Dogs. It passes Island Gardens DLR station before heading back north again along Manchester Road, crossing another couple of dock entrances.
The road meets the A1261 again at a roundabout GSJ above the Blackwall Tunnel (which is not accessible from here) before ending slightly further on at traffic lights on the A13.