[2] At its southern end, the A400 begins in the City of Westminster at a junction with the Victoria Embankment, opposite the Hungerford (Charing Cross) Bridge.

There is no vehicular access to the bridge, which provides pedestrians with a route over the River Thames to the South Bank.

After Warren Street station, the A400 becomes a dual carriageway, part-way along Hampstead Road, as it passes through Somers Town.

Hampstead Road's northern terminus is at Mornington Crescent tube station, and northbound A400 traffic travels along Camden High Street to the Britannia Junction.

[4] The route enters the London Borough of Islington and becomes Junction Road between Tufnell Park and Archway, before it meets the A1.

Levels of Nitrogen dioxide (NO2) sometimes exceed the Air Quality Objective, set by the UK Department for Environment, Food, and Rural Affairs (Defra) at 40 micrograms per metres cubed (μg/m3).

In 2018, levels of NO2 failed to meet the Air Quality Objective, with the monitoring point recording an annual mean of 76 μg/m^3.

[6] Results from 2018 include: In the London Borough of Islington, a monitoring point along Junction Road recorded an annual mean 36 μg/m^3 in 2018, down from 42 μg/m^3 in 2017.

For most of its length, Hampstead Road is sandwiched between Regent's Park Estate to the west and Euston railway station to its east.

In addition, from south to north on Hampstead Road are Euston Tower, the Camden People's Theatre, Drummond Street, the former Maria Fidelis Roman Catholic Convent School (recently relocated due to HS2), the Prince of Wales pub, the St Pancras Female Orphanage, and the High Speed 2 terminus under construction at Euston (formerly site of the National Temperance Hospital).

Hampstead Road then crosses the West Coast Main Line, before forming the western boundary of the Ampthill Square Estate and the western side of Harrington Square, passing on the left the Carreras Building, and on the right Mornington Crescent tube station, then joining Camden High Street at the statue of Richard Cobden.