|East end||A484 (Loughor)|
| A484 road|
|West end||M4 Motorway Junction 47 (Penllergaer)|
The A4240 begins at the roundabout at the Loughor side of the Loughor bridge. The road heads east through Loughor, then onwards through Gorseinon, where it forms the east-west main road intersecting with the B4296 in central Gorseinon. The A4240 then continues through the Garngoch Industrial Estate and then through Penllergaer after which it continues along a single carriageway shared with the A48 until it reaches the roundabout which forms Junction 47 with the M4 Motorway.