The A1023 starts at junction 28 of the M25 (also known as Brook Street Interchange), and then heads northeast as Brook Street into Brentwood. While Going through Brentwood the road changes name to London Road, High Street, then Shenfield road, where it meets the A128 at Wilson's Corner junction. At Wilson's Corner northbound traffic follows Ongar Road towards Chipping Ongar and the A414, whilst southbound traffic follows Ingrave Road towards Ingrave and then on to the Southend Arterial Road (A127). The road then continues northeast from Wilson's Corner, where it meets with the western end of the A129, which goes towards Sheffield station Hutton, and Billericay. It then continues as Chelmsford Road until it meets the A12 again at the Maryland Interchange.
