It runs between the A1 at Markham Moor, Nottinghamshire and Chain Bar Interchange – Junction 26 of the M62 motorway, south of Bradford in West Yorkshire.

The route starts off as a non-trunk road at the Markham Moor interchange (previously a roundabout before 2008) in West Drayton.

When the West Drayton diversion opened in February 1957 (including the roundabout) as the A57, the A1 still went through Retford, until August 1961.

It enters the White Houses part of Retford as London Road, with a right turn for Nether Headon, passing The Whitehouses and the JET Whitehouses Service Station, a left turn for Ordsall, then crosses the East Coast Main Line.

It crosses the Sheffield to Lincoln Line, passes the former King Edward VI Grammar School on the right (now the Retford Oaks High School on a new site) opposite the Esso King Edwards Service Station, and there is a left turn for Albert Road (B6044), the main access for Retford railway station.

It crosses the Cuckoo Way and the Chesterfield Canal as Arlington Way, passes the Sherwood Centre, the Bassetlaw Museum,[1] and St. Swithun's church, and the A620 leaves to the north (for Gainsborough) at traffic lights.

It crosses the River Ryton, joins the A614, a trunk road, at an awkward junction at Harworth Bircotes.

At Warren House Farm near Hurst Plantation, there is a main right turn for the airport, where the A638 enters the parish of Rossington, deviating from the Ermine Street to the west.

It meets the A18, from the right, at a roundabout next to the Grand St Leger Hotel, and the former Great North Road continues straight ahead as South Parade through the town centre.

The road used to leave at the junction next to the Frenchgate centre, (The old North Bridge, it has been reopened for outgoing traffic, incoming traffic is still restricted to public transport, taxis and emergency services).As Church Way it passes St George's Minster, Doncaster and leaves at the next roundabout, at the start of the A19 next to the new Doncaster College.

It crosses on St George's Bridge over the River Don and the East Coast Main Line.

At a large roundabout, it rejoins the former Ermine Street, and the A19 leaves to the north-east where a railway line crosses over the road.

As York Road the Centurion Retail Park and a Morrisons is on the right, and the JET Royal Blue Garage on the left.

It turns due west and enters Ackworth Moor Top, meeting Barnsley Road (A628) at a roundabout next to the Beverley Arms.

It crosses a railway line, and at Warmfield cum Heath it meets Black Road (A655) from the right.

Entering the town of Wakefield as Doncaster Road, it passes under a large railway bridge.

It crosses the River Calder near the Chantry Chapel of St Mary the Virgin, Wakefield, and the A61 leaves to the north as Kirkgate and as the dual-carriageway Ings Road the A638 passes under a railway, carrying the Hallam Line and Huddersfield Line, then passes retail parks and meets Denby Dale Road (A636) at a roundabout next to the Royal Mail sorting office.

It passes the Shell Ossett and Roundwood Industrial Estate on the left, and becomes a dual-carriageway, built with the M1 junction 40 by Costain in October 1968.

At Earlsheaton there is a crossroads, and the road descends down the Calder valley into Dewsbury passing the BP Ossett Service Station.

The untrunked section passes Dewsbury railway station and meets Leeds Road (A653).

It becomes Westgate and passes the imposing Flush Mills, home of Rieter Automotive, who make car furnishings.

It passes a BP (former Shell) Millbridge Filling Station on the left, and in Littletown it becomes a dual-carriageway, which destroyed much of the village centre when it was built.