The route of the current A2 follows a similar route to that of a Celtic ancient trackway. It was an important route for the Romans linking London with Canterbury and the three Channel ports of Rutupiae (now Richborough), Dubris (now Dover) and Portus Lemanis (in modern Lympne). It had river crossings at Rochester over the River Medway; Dartford (River Darent) and Crayford (River Cray). The Romans paved the road and constructed the first Rochester Bridge across the Medway. Access to London was via London Bridge which was first constructed by the Romans in AD 50. The road was known as 'Item a Londinio ad portum Dubris' and appeared in the Antonine Itinerary, a contemporary map of Roman roads in Britain.
In Anglo-Saxon times it became part of a longer road known as Wæcelinga Stræt ('Watling Street' in modern English). Rochester Bridge was rebuilt in stone in 1391 and the 'Wardens and Commonalty of Rochester Bridge' were created to maintain the bridge, the modern equivalent of which, the Rochester Bridge Trust, still manages the current crossing.
By the 17th century the road had fallen into disrepair and sections were converted into turnpike roads by various Turnpike Acts passed by Parliament. The section from Gravesend and Rochester was turnpiked in 1712 and then from Chatham to Canterbury in 1730; the section from there to Dover was only turnpiked in the 19th century by which time it was known as the Great Dover Road. Rochester Bridge was rebuilt in cast iron in 1856 (and rebuilt again in 1914 with a second bridge in 1970). The London, Chatham and Dover Railway completed a railway route from London to Dover in the 1860s. Access to London from the A2 across Thames was improved with the completion of Tower Bridge (1894), Blackwall Tunnel (1897) and the Rotherhithe Tunnel (1908).
The road was given the reference A2 within the Great Britain road numbering scheme in the 1920s. The Dartford Southern By-pass, which relieved chronic congestion on the old route through Dartford (along what is now the A207) provided considerable local employment during the Post–World War I recession. It was opened by the Prince of Wales in 1924. It is estimated that the project provided 63,500 days work to local unemployed people.
The Dartford Crossing opened as a single tunnel in 1963 (then as a dual tunnel in 1980) and provided additional traffic on the A2 and the M2 Motorway opened in stages between 1963 and 1965 providing an alternative faster route for long-distance traffic avoiding the A2 through Rochester and Chatham. The 3-lane stretch of the A2 between Falconwood and Cobham was built in stages from around 1963 until 1973. A section of Ringway 3 (part of the proposed M16 motorway linking to the A2, which later formed part of the M25 motorway opened between 1974 and 1977.
The Rochester Way Relief Road, was opened in 1988. The M20 motorway to Folkestone was constructed in stages from the 1960s and provided a faster route to Dover following extensions built in the early 1990s after which traffic levels on the M2 and A2 reduced. The Queen Elizabeth II Bridge opened in 1991.
A new flyover for A2 Westbound to M25 Northbound traffic was completed late 2007.
In 2008, a section of the A2 beside Gravesend was widened to 3/4 lanes between Pepperhill (suburb of Northfleet) to Cobham. It was also re-routed away from the houses of Gravesend/Singlewell to make room for the new lanes and reduce the amount of noise from the widened roads. The old route of the A2 (the Roman road) has been made into a footpath/cyclepath. The new road was opened in 2009. A section of the old road has been turned into Cyclopark, with footpaths, cyclepaths and an equestrian route along the old road.
A new Lower Thames Crossing down river from the Dartford Crossing linking to the A2 has been proposed by the Department for Transport in 2009.
Its original alignment roughly followed a mix of the ancient Celtic route and the turnpike road to Dover. The Roman alignment, however, is not easy to identify and much of the original A2 does not exactly follow what is known of the Roman route (the straightness of many long stretches is misleading). A section of the modern A2 from Rochester to the Roman settlement of Vagniacae, modern Springhead, is believed to roughly follow the Roman route.
The A2 starts at Borough in Central London, at a junction with the A3, near the church of St George the Martyr. The remains of a small Roman temple was excavated at Tabard Square in 2003. The A2 at this point is named Great Dover Street, and is the only part of the A2 within the congestion charging zone. At the end of the road, it meets the London Inner Ring Road and becomes a primary route. The A2 heads along Old Kent Road towards New Cross, where the A20 breaks away. The A2 continues east through Deptford and Blackheath until it arrives at the Shooter's Hill Interchange with the A102 near Greenwich. A section of the Roman road has been identified running through Greenwich Park on an alignment with Vanbrugh Park.
At Shooter's Hill, Watling Street and the A2 part company, as Watling Street continues along the A207. At this point the A2 joins a dual carriageway, the Rochester Way Relief Road (the carriageways to the north of this junction being the A102) with a 50 mph (80 km/h) limit.
The A2 meets traffic lights at Kidbrooke; this is the last set of lights out of Greater London (the lights only affect right-turns coast bound). Shortly after here, the A2 meets the South Circular Road and becomes a motorway in all but name, with 3 lanes and a hard shoulder in each direction. Within the London section the left hand lane is used for local traffic.
At Falconwood, the road becomes the East Rochester Way - this point was once the westbound terminus of the dual carriageway. There are exits for Bexleyheath, Black Prince (at Bexley), and other south east London suburbs. Motorists should be aware of the numerous speed cameras along this stretch. Just before the A2 reaches the junction with the A2018 the speed limit increases to the national speed limit (70 mph, 113 km/h) and the road heads east into Kent towards Dartford, bypassing the town to the south and cutting through Dartford Heath. The next junction links the road to the M25 London Orbital Motorway at Dartford; the next, (Bean Interchange), is for the B255 and A296 for Bluewater, where Watling Street rejoins the A2.
The A2 then bypasses Gravesend, before merging into the M2 at Three Crutches Interchange, near Strood.
This latter section, from the Pepper Hill Interchange (near Northfleet and the Ebbsfleet International railway station), to the Cobham Interchange underwent extensive works (2007–2009) to move the entire carriageway south to run alongside the Channel Tunnel Rail Link, so as to remove the road away from local housing - and to continue with a fourth lane in each direction, with grade separated junctions.
The A2 reverts to a non-primary single carriageway road at this point. The M2 now parts company to bypass the Medway Towns, while the A2 heads into them. It enters Strood before crossing the River Medway into Rochester. The A2 bypasses the old High Street, heading instead along Corporation Street alongside the railway lines. It crosses the old High Street, climbs Star Hill and heads east into Chatham where it meets the A230 and A231 by way of a one way ring road. The A2 heads to the south of Gillingham, where the A289 Medway Northern Bypass joins it. The A278 departs from the A2 to head south to meet the M2, whereas the A2 goes through Rainham and Newington before entering green country for the first time since Three Crutches.
The road meets the A249, now dualled, to the west of Sittingbourne, before going through the town itself (bypassing the now-pedestrianised High Street, as it does in Rochester and Chatham). Leaving Sittingbourne, the A2 continues east in almost a straight line, for it is still along the alignment of Watling Street at this point. At Ospringe it passes the Maison Dieu, now a museum of Roman artifacts but originally a wayside hospital commissioned by Henry III in 1234. The A2 continues to Faversham, but doesn't (and never has done) go through the town, preferring to almost bypass it to the south. A mile (1.6 km) later, it arrives at Brenley Corner, junction 7 of the M2.
At Brenley Corner, the A2 once again becomes a primary dual carriageway. Straight after the interchange, Dunkirk and Boughton are bypassed as the A2 continues towards Canterbury and Dover. Formerly it went through Canterbury city centre, this has now been bypassed and the original route is now called the A2050. It breaks off from the A2 at Harbledown - just after Gate Services - taking all Canterbury traffic with it. At Wincheap, the A28 meets the A2, although only westbound traffic can exit the A2 at this point. The A2050 rejoins the A2 at Bridge.
Traffic for the Channel Tunnel and Folkestone leaves at the junction with the A260, and shortly after this junction the A2 loses its dual carriageway status, regains it, and loses it again within a few miles. Now single carriageway, the A2 forms the Dover bypass. It meets the A256 for Sandwich and Ramsgate at a briefly dualled section, before reverting to single carriageway for the final time. The A2 breaks through the cliffs above Dover Docks before turning 180 degrees by way of a pigtail bridge and "touching down" on land again at the entrance to the Eastern Docks, where both it and the A20 terminate.