The 18hp was introduced in 1921. This was a 2,318cc scaled down version of the 30hp. A Connaught-bodied 18hp, which weighed in at two tons, was also sold to The Duke of York. In a letter, his Royal Highness complained that the car was rather sluggish. It was suggested he put the body on a 30hp chassis. It is often very difficult to tell from advertisements and photographs which model was which, as both were large vehicles. In April 1923, a 30hp Pullman Limousine was built as the honeymoon car for the wedding of the Duke of York and Elizabeth Bowes-Lyon. The company's advertising capitalised on this with views of regal cars in Royal parks.

In 1923, the 14hp four-cylinder model was introduced, a small Armstrong Siddeley, it was a medium-sized car by anyone else's standards. Over 14,000 were built over six years and many of these were exported. This robust model was certainly the most numerous of the company's output during the vintage era and, with its flat radiator and high clearance, it proved very popular in rugged terrain.

Early in the 1920s, W G McMinnies took over the job as publicist for Armstrong Siddeley Motors. He then composed a series of slogans that emphasised the quality of the product. Examples of these included: "Finest value in every class", "Can you stop?" (advertising four-wheel brakes) "Reputation-raise the bonnet", "My lady must ride in a beautiful carriage", "Travel in comfort in the blue train of the roadway" and "You cannot buy a better car". This form of hyperbole tended to reinforce the truth rather than hide it, as the product justified all the claims.

In October 1925, the MkII models were introduced with 14hp 18hp and 30hp chassis. The two smaller cars were given new chassis layouts, four-wheel brakes became standard and the 30hp went from a biblock to a monoblock engine. In 1927, a 15hp model was introduced. This ran alongside the 14hp model and was similar in design except for a six-cylinder side-valve engine, but developed into a totally different vehicle. In 1928, the Wilson-Epicyclical fluid flywheel drive was introduced. This semi-automatic gearbox was a great improvement on the crash box of the day, and was part of a new trend in semi-automatic transmissions.

In 1931, the V-radiator was introduced on the 15hp and by 1932 on the 12hp. The 12hp had been introduced in 1928 as a fabric-bodied saloon based on the Riley Monaco. In 1931, its engine was enlarged to 1434cc side valve six. The 1930s Armstrong-Siddeley house style was established throughout the range, and changed only in detail prior to the Second World War.

In 1932, a "new" 20hp chassis was introduced with Sports, Saloon, Limousine, Coach Saloon and Tourer bodystyles. With a new chassis layout and an uprated engine, this car had a good performance. Three short chassis Rally tourers won Glacier Cups in the 1932 Alpine Trial and their Wilson gearboxes were of great interest. At the 1933 Motor Show, the Siddeley Special was introduced as a replacement for the old 30hp, with the same cubic capacity of 4960cc. The engine was largely of aluminium alloy and as a result it was capable of 100mph in tourer form - and much was made of Malcolm Campbell's ownership of one in the company's advertising. It was also available in a variety of proprietary coachwork styles.

Armstrong Siddeley entered the Rainbow Cars in the 1934 RAC Rally, a team of twelve vehicles painted in all the colours of the rainbow, including three short chassis Sports Tourers, one of which was driven by Cyril Siddeley. The 12hp coups chassis was marketed for "The Daughters of Gentlemen", a successful ploy as at least a third of this model's production run was sold to women!