In 1909, J D Siddeley took over as General Manager of the Deasy Motor Co. later becoming Managing Director. Old designs were carried over until new designs could be constructed. New cars were marketed as JDS-Type Deasys, because the Siddeley name was subject to legal proceedings between Deasy and Wolseley. This was resolved by 1912 and Siddeley was then allowed to use the name Siddeley-Deasy freely.

The Deasy car disclosed Deasy's Irish heritage. The top part of the radiator shell was based on the shamrock showing the outline of the shamrock-shaped radiator with an intertwined "DC" for Deasy with the name underneath. This remained until 1912 when the logo became an entwined "SD" in a circle.

In November 1913, the Sphinx made its first appearance. Before manufacturing new models, a Renault was bought in and minutely examined in the factory. The resulting JDS type-Deasys had a scuttle-mounted radiator and a coffin shaped bonnet. Local wags called the vehicle a coffin on wheels. Siddeley was not amused, but when a journalist described the car in 1912 as "quiet and inscrutable as the sphinx" alluding to the curious bonnet style and the "Silent-Knight" sleeve-valve engine, Siddeley saw an opportunity and the Sphinx became both logo and mascot, continuing until 1960.

Various engines were used including Aster, Daimler and the Daimler-built Knight sleeve-valve. At one point, one model had a Rover 12hp chassis. A wide range of chassis and engine sizes were available up to 1914. One car planned for 1914 but not built was a 30hp car with a bi-block six-cylinder engine of 4960cc.

When the Great War started in August 1914, Siddeley Deasy was a successful medium-range car builder, then things changed dramatically. By the end of the war the company's workforce had increased tenfold, from 500 to 5,000. With patriotic fervour, Siddeley encouraged the workforce to enlist. Not long after, with substantial orders for lorries, ambulances and staff-cars, he reversed this policy.

In 1915, Siddeley persuaded his board of directors to invest in tools and expand to produce airframes and aero engines. Airframes were built for the RE7 and RE8 Aero engines, including the V8 RAF 1 A and the air-cooled V1 2 RAF 4A. The development work then began on a six-cylinder in-line water-cooled BHP aero-engine. To achieve maximum reliability Siddeley had to completely re-design this engine, which was developed into the Siddeley-Deasy Puma engine. Over 6,000 were built. Pressures to produce this engine for the DH9 bomber resulted in production commencing at too early a stage in development and it proved underpowered. However, it was quite satisfactory for post-war use.

After the war there was a shortage of materials. To ensure adequate supplies of steel, Siddeley Deasy merged with the Sir W G Armstrong's Whitworth company. Siddeley became Managing Director of Armstrong Siddeley Motors, but was not on the main board of Armstrong Whitworth Development Co.

The first car manufactured by this new company was the 30hp. This was none other than the 4960cc bi-block six designed as a Siddeley Deasy in 1914. Changes to the car at this stage included a V-fronted radiator, believed to have been revamped by the renowned automotive artist Frederick Gordon Crosby. A little earlier the company was said to have imported a Marmon car from the USA, stripping down the vehicle to its component parts. Ernest Siddeley when asked, said it was not dismantled in the Billiard room, (they did not have one), but Ernest did visit the Marmon factory in the USA in 1919 and was more than aware of American advances in car manufacturing.

The 30hp was sold to the European aristocracy and was popular with the bespoke carriage trade. One loyal patron was the then Duke of York, who owned at least six different examples before he became King George VI, the first of which was a 30hp which he went to the factory to collect.