The route is one of the oldest in London, going back to at least 1851, along with the routes 9 and 11. In February 1907, the first bus produced by British Automobile Development Co. and Brush Electrical Engineering Co. Ltd. went into service on this route. Tilling-Stevens petrol electric buses were used on route 12, running between Oxford Circus and Peckham. In the 1930s destinations between South Croydon and Brent Cross station were served. By the 1950s, the route operated between South Croydon and Willesden Junction, although buses from Croydon generally ran no further than Oxford Circus and those from Harlesden no further than Dulwich. The route has been run from a number of garages. The computer scheduling system developed by London Buses in the 1970s is designed for a maximum of four garages operating on any one route; this was derived from route 12, which had four garages operating vehicles on it at the time. During the annual Notting Hill Carnival express services operated as 12X. Route 12 was shortened to terminate at Norwood Junction in 1972, with new route 12A taking over between South Croydon and Peckham. On 25 October 1986, the route was reduced to operate only between Penge and East Acton. From 14 August 1988, route 12 was curtailed south of Dulwich Library, capacity being maintained by extending route 78 from Dulwich to Forest Hill and route 176 from Forest Hill to Penge. The western terminal was cut back to Shepherds Bush on 13 July 1991 and to Notting Hill Gate on 12 March 1994. The route was operated by AEC Regent III RTs until replaced by AEC Routemasters in 1973. The route was restructured on 5 November 2004, with the section between Oxford Circus and Notting Hill Gate withdrawn, with route 390 extended. At the same time the Routemasters were replaced by Mercedes-Benz O530Garticulated buses. Free WiFi and passenger information screens were introduced in August 2014 on two buses as part of a trial by Transport for London. Wright Eclipse Gemini 2 bodied Volvo B9TL and Wright Eclipse Gemini 2 bodied Volvo B5LHdouble deck buses were introduced on 24 November 2011 as part of the Mayor of London's policy to withdraw articulated buses from London. London Central has successfully retained route 12 with new contracts starting on 6 November 2004 and 5 December 2011. New Routemasters were introduced on 28 March 2015. The rear platform remains closed at all times except for when the bus is at bus stops.
