Scammell began when the wheelwright, George Scammell of Spitalfields, London developed his business. Later on when the name of the company became G Scammell & Nephew, they were involved in the building and repair of craftsman built carts and vans. In the early 1900s, they had built a substantial business by selling and maintaining Foden steam wagons and small trucks.

The outbreak of war in 1914 presented itself as a turning point in road transport history. Mechanical transport was seen to work, proving its vast potential beyond doubt to such forward thinking companies such as Scammell.

George Scammell's great nephew, Lt Col Alfred Scammell was injured and invalided out of the army and he was able to apply the practical experience he had gained during the war and began developing the articulated six wheeler, which began production in 1920. This vehicle was articulated and its very low axle weight allowed it to carry 7½ ton payload at 12 mph rather than being limited to 5mph. In 1921, a test vehicle pulled a load of just under 8 tons up West Hill in Highgate in second gear and managed 18mph on the flat.

By 1927, Scammell had strengthened their position by launching its first cross-country vehicle that was called the Pioneer. It was a 6x4 rigid, which had a walking beam bogie so that any of its four wheels could be raised two feet without losing traction. The mobility could be further enhanced by adding a driven front axle.
The early 1930s saw the production of the 3 wheeled 'Mechanical Horse', designed by Oliver North to replace horses in rail, postal and other delivery applications. It featured automatic carriage coupling and the single front wheel could be steered through 360 degrees. It was sold in 3 and 6 ton versions. A 1125cc side valve petrol engine powered the 3 ton version and a 2043cc engine powered the 6 ton.

Scammell were top of the range, expensive and built in small quantities. The depression affected the firm badly and it was in a poor financial position in 1934. Watford Council had helped by ordering a pair of fire engines, but it was not enough, and it was Shell-Mex that injected capital, but in return, they insisted on a management shake up which saw Alfred Scammell deprived of his Managing Director post.

During the Second World War, Scammell made a massive contribution to the war effort by building large numbers of tank transporters, gun tractors and heavy recovery vehicles as well as fire pumps.

In the late 1940s, Scammell produced the Scarab, which replaced the 'Mechanical Horse'; the Scarab had similar features but had a less angular cab and a 2090cc engine in both models as well a diesel version, which featured a Perkins engine.
In 1955, Scammell became part of the Leyland Group and this provided ready access to the Leyland engines, gearboxes and axles. A gradual replacement for the lightweight range by new models using Leyland engines were the 4x2 Highwayman MU, the Routeman 8 wheeler and a 4x2 forward control MU called the Handyman.

The beginning of the 1960s saw the introduction of the new Michelotti designed glass fibre reinforced plastic cab for the Routeman, Handyman and the new twin steer Trunker.
The divisional reorganisation within the Leyland Group had resulted in the name becoming Scammell Motors, and the closure of Transport Equipment (Thornycroft) in 1972 resulted in the Thornycroft Nubian range transferring to Scammell, together with the LD55 dump truck.

The late 1970s saw a surge in development and the birth of the Contractor Mark 2 heavy hauler with an 18 litre, 425 hp Cummins engine and automatic gearbox and the first Commander Tank Transporter for the British Army were produced.

Two developments by Leyland Motors at the end of the 1970s benefited Scammell. Leyland wished to develop two new heavy vehicle ranges, the overseas bonneted Landtrain and the UK forward control Roadtrain that would feature the new C40 tilt cab.
In view of Scammell's expertise, Leyland tasked them with the development of the Landtrain and they were able to use the same cab and bonnet for the replacement for the Contractor. The new range, the S24 was available in 6x4 and 6x6 formats. The full weight range was 40 ton to 200 ton GTW. Leyland also entrusted the 8 wheeler version of the Roadtrain, called the Constructor 8 to Scammell and this, at last gave Scammell access to a modern tilt cab.

A military 6x6 version with a Rolls Royce 350 engine, ZF automatic gearbox and Kirkstall axles followed, and this was offered in 8x6 form to the British Army in 1986 for the hooklift equipped DROPS vehicle requirement. In 1987, Scammell learned that the tender for 1522 such vehicles was successful, but also that the Leyland Group had been purchased by DAF BV of Holland.

DAF elected to build the DROPS and selected S26 vehicles at the Leyland Plant and planned to close the Watford factory.

In July 1988, the Watford factory closed. The S24 and the Nubian ranges, together with the rights to the Crusader and Commander were sold to Unipower Ltd, who opened a new plant in West Watford.