The A249 begins close to Maidstone town centre, heading eastwards from the southbound A229 Lower Stone Street along first Mote Road and then along Wat Tyler Way. Due to Maidstone's one-way system, the westbound carriageway extends for a short way along Knightrider Street (towards the Archbishop's Palace, Maidstone) but then ends where it meets the northbound carriageway of the A229. At the top of Wat Tyler Way, where that road meets King Street, the A249 occupies a small gyratory system that joins the A20 (heading east to Ashford).

From there, the A249 heads north on Albion Place. This very quickly becomes the Sittingbourne Road, which crosses over the railway line between Maidstone and Bearsted. After passing Vinters Park, it reaches the north end of the town at the Chiltern Hundreds roundabout (named after the adjacent pub). This old route between Maidstone and Sittingbourne is broken, as the old Sittingbourne Road has become a park and ride facility. This break occurred during the 1990s, as the result of works to widen and upgrade the former A20(M) Maidstone Bypass section of the M20 to remove a bottleneck on the newly completed motorway.

Instead of that old course, the route now turns east onto the Bearsted Road towards Grove Green, becoming a dual carriageway, before turning north again just before Newnham Court Farm. There, the route crosses the M20, rejoining the Sittingbourne Road at the exit of the motorway roundabout. From this point onwards, the route becomes primary.

The road then climbs up onto the North Downs via Detling Hill, past the village of Detling and crossing the Pilgrim's Way and North Downs Way (between Boxley and Thurnham). Just after the M20 junction, where it meets Pilgrim's Way, there is a footbridge called Jade's Crossing, named after a schoolgirl who died along with her grandmother in a road accident in 2000 whilst attempting to cross the busy road in the absence of such a bridge.^[2] Immediately after the footbridge, the carriageways diverge, with the northbound carriageway taking a higher route up the hill and the southbound carriageway taking a separate lower route that includes a chicane at the bottom of the steep hill.

After Detling Hill, the road becomes known as the Maidstone road and passes the Kent County Showground (on the left) and White Horse Wood (on the right). Further on, the road passes a large lay-by on the northbound carriageway, which was closed in 2005 after local residents complained that it was a haunt of doggers.^[3] The road passes the village of Stockbury (on the left) and then arrives at a junction with the M2.

From there, the route used to carry on as a single carriageway towards Sheppey on the old Maidstone Road. However, in the mid-1990s, a new, dualled Maidstone Road running parallel but slightly to the west was completed, running under the M2 viaduct. The route then passes the hamlets of Danaway and Chestnut Street. The next junction on the A249 comes where it goes under the A2, with exits to a roundabout above that leads to Newington and Rainham to the west or to Sittingbourne to the east.

Past the A2, the route continues across the flat plains of Swale, past Bobbing (to the west), Kemsley and Milton Regis (to the east). As it approaches Swale railway station on the southern banks of the Swale, the road becomes the Sheppey Way, which crosses the Swale via the Sheppey Crossing, opened in 2006,^[4] having previously used the Kingsferry Bridge, which lifts to allow ships to pass along the Swale; non-motorised traffic still uses the old route and rejoins the A249 on the island.

On the Isle of Sheppey, the road follows a new dual carriageway built in conjunction with the new bridge as far as Queenborough. Then it returns to a single carriageway along Brielle Way into Sheerness, past Blue Town (on the left), before ending at a roundabout near the seaport and former passenger ferry port in Sheerness. There, the A249 becomes the A250 on Bridge Road.

In 2020, Kent County Council installed mobile CCTV cameras along the A249 in anticipation of HGV backlogs due to Brexit.^[5]

From September 2021, National Highways are carrying out improvements to J5 of the M2 at Stockbury, which will involve the construction of a new flyover of the roundabout for A249 traffic heading through the junction, and new slip-road connections between the M2 and the A249.^[6][7] Works to realign the slip roads leading from the A249 to the eastbound carriageway of the M2 were completed in January 2023.^[8] New local road links are also being built as part of the scheme. The entire scheme is now scheduled for completion in early summer 2024.^[9]

• Highways Agency – Iwade-Queenborough improvements
• Highways Agency – Iwade-Queenborough improvements (photos)

51°20′33″N 0°41′13″E / 51.3426°N 0.6869°E / 51.3426; 0.6869 (A249 road)