Part of the A40 crossed the Welsh border just west of Dean Forest Park. When taken south-west, it ran through Monmouth, where it intersected with the A466 and crossed a branch of the River Wye. The A40 curved west, with the A449 branching south just north-east of Raglan. Continuing on the A40, it curved north-west to reach Abergavenny where it intersected with the A465. The A40 then continued north-west into a national park, following the River Usk as it passed through Crickhowell.
Part of the A40 followed the Afon Tywi westwards, then curved south into Carmarthen, where the A48 branched east. The A40 continued west, before curving south-west and crossing a railway. It then branched off into two directions.