After the success of the Austin A105 Vanden Plas, Sir Leonard Lord requested Vanden Plas to make a quality version of its replacement the A99 Westminster and its sister the Wolseley 6/99.

Two prototypes were completed by March 1959. The first was a saloon model based on the Austin A99 and the other a Touring Limousine (with interior glass division) was based upon the Wolseley 6/99. To distinguish them from the A99 and 6/99 they were called the Austin A120 Vanden Plas and the Wolseley 6/120 Vanden Plas.

As can be seen the Austin received a different grille from the A99 similar to the later A110 Westminster grille. It also had different front side/flasher lights, a small chrome coronet on the front bonnet and stainless steel sill strips. The hubcaps had a centre 'P' motif and the boot lid a Vanden Plas script badge as well as an A120 badge. Internally the A120 received a completely different and elegant walnut covered facia with recessed instruments. The Vanden Plas designed seats were covered in leather and walnut capping's were fitted to the window sills.

The Wolseley 6/120 received different wing mouldings than the 6/99 plus Vanden Plas  and 6/120 scripts on the boot lid.  The hubcaps received coronet central badges rather than the 'P' of the Austin. Internal differences were once again the facia, door cappings and leather covered Vanden Plas seat design. 

Production of the A120 and 6/120 Vanden Plas models commenced in May 1959. However after one week, and after only 5 of each model had been made, Lord requested Vanden Plas to cease production and to design a 'Princess' 3 Litre. 

None of the 10 vehicles produced is believed to survive.