The A1 in London is the southern part of the A1 road. It starts at Aldersgate in the City of London, passing through the capital to Borehamwood on the northern fringe of Greater London, before continuing to Edinburgh. The road travels through the City and three London boroughs: Islington, Haringey and Barnet, which include the districts of Islington, Holloway, Highgate, Hendon and Mill Hill, and travels along Upper Street and Holloway Road, crossing the North Circular Road in Hendon, a district in the London Borough of Barnet.

The A1, also known as the Great North Road, is the longest numbered road in the United Kingdom, at 410 miles (660 km). It connects London, the capital of England, with Edinburgh, the capital of Scotland. The numbering system for A-roads, devised in the early 1920s, was based around patterns of roads radiating from two hubs at London and Edinburgh. The first number in the system, A1, was given to the most important part of that system: the road from London to Edinburgh, joining the two central points of the system and linking the UK's (then) two mainland capital cities. It passes through or near north London, Hatfield, Welwyn Garden City, Stevenage, Baldock, Letchworth Garden City, Biggleswade, Eaton Socon, Buckden, Peterborough, Stamford, Grantham, Newark-on-Trent, Retford, Doncaster, Pontefract, York, Wetherby, Ripon, Darlington, Durham, Gateshead, Newcastle upon Tyne, Morpeth, Alnwick and Berwick-upon-Tweed.