The road starts in Churchstoke at a junction with the
A489. From here it heads north, crossing over the border into Shropshire, and passes through
Chirbury. After Chirbury it crosses back into Powys and past
Forden. It then crosses over the
River Severn and heads by
Welshpool Airport. It crosses the
A483 approximately 1 mile south of
Welshpool and then runs through the town's western suburbs before joining the
A458 in the town centre. The two A-routes run together for less than a mile; at Raven Square roundabout the A490 splits off from the A458 and continues north. It crosses the
River Vyrnwy before joining for a short distance the
A495. A few miles after splitting off from the A495 it reaches Llanfyllin, and shortly after ends by splitting into the B4391 and
B4393 roads.