Millbank Interchange forms junction 12 of the M74. It gives access to the A70, B7078 and Happendon services from the south, while junction 11 gives access from the north.
It was built in 1987 as the southern terminus of the first southern extension of the M74 (see M74 and A74(M) History). Until then the A74 and A70 had an at-grade staggered junction comprising two T-junctions with central reservation gaps. The A74 was then on roughly the line of the present southbound on-slip; the eastward A70 met it at the site of the present eastern roundabout; the westward A70 met it just north of where today the slip road joins the mainline. Remains of the westward A70 can still be seen.
The 1987 extension of the M74 was generally to D2M standard, but, as a late alteration to the plan, a climbing lane was added from the A70 bridge southward. Up to the end of the motorway this was done by abolishing the hard shoulder; on the all-purpose A74 south of the slip road merge, it was done by actual widening.
In 1991 the M74 was extended southward from here at D3M standard. For the first mile south from the junction, the motorway was an on-line widening of the A74, so a new stretch of parallel B7078 was built to the west to maintain access for non-motorway traffic. It remains the case that the southbound carriageway lacks a hard shoulder from the A70 bridge to the slip road merge point.
When the southward M74 upgrade was first completed, the 3-lane downhill northbound carriageway lost its left lane into the northbound exit slip road, which then met the A70 at a simple T-junction. It is not known whether any incidents resulted but after about a year the layout was altered. The northbound carriageway was reduced to two lanes by using road markings to close the right lane almost two miles before the junction, and a small roundabout was built where the slip road meets the A70.