The road is approximately 16.8 miles (27.0 km) long and it runs from Orsett, at its southern origin, to Chipping Ongar at its north end.

At its southern end, the A128 originates as an exit from the Stanford-le-Hope bypass of the A13 near Orsett, where it is named Brentwood Road.

As the road continues in a northbound direction, motorists pass through the villages of Herongate and Ingrave (where the A128 becomes known as Ingrave Road) before a junction at the eastern end of Brentwood High Street.

At this double mini-roundabout, called Wilsons Corner, drivers can turn onto the A1023, westbound towards Brentwood or eastbound towards Shenfield.

It terminates at the Four Wantz roundabout where motorists can take either the A414 (westbound towards Harlow or eastbound for Chelmsford) or the B184 toward Fyfield.