The Thornycroft era began in 1862, when John Isaac Thornycroft designed a steam car. Two years later, he formed the Steam Carriage and Wagon Company, with works at Chiswick, London. However, the project stopped because of over-zealous legislation for road vehicles, and Thornycroft took up shipbuilding, also at Chiswick.

The company took up road vehicle work again in 1895, whilst continuing with the shipbuilding and marine engineering business, and built his first steam vehicle at the Chiswick works. The Thornycroft Steam Wagon Company of Chiswick put steam lorries and vans into production, and a new factory was set up at Basingstoke, Hampshire in 1898, to meet demand.

In 1901, the war office held a competition and attracted by the possibility of orders and a £500 prize for the winning vehicle, several firms, including Thornycroft, entered the competition held at Aldershot for the best type of powered vehicle for military use. The winner was Thornycroft's steam lorry, which was followed in second place by a Foden lorry - also steam driven.

The first Thornycroft motor vehicle was introduced in 1902, a commercial vehicle with a 4 ton load capacity. The firm gave up building cars in 1912 to concentrate, for the next half century, on building its successful commercial vehicles; a field in which Thornycroft became world-class and and a major employer in Basingstoke.

In 1909 the authorities held a competition for the best IC engined tractor. Many leading firms entered this competition, which lasted several days, largely over difficult terrain, and a £750 prize was awarded to Thornycroft's entry, Thornycroft Vehicle Number 833 - the only vehicle that was deemed worthy of an award!

After World War 1, Thornycroft continued production of its four proven pre 1919 models, including the 2 ton BT, 3 ton X, 4 ton 40hp J and 5 ton 40hp Q type models. Later on, these types were joined by the 6 ton 40hp W, Thornycroft's first post war lorry design.

Thornycroft had become a major firm whose extensive product range included motor vehicles for goods, passenger transport and municipal service, cargo and passenger ships up to a length of 450ft, shallow draft vessels, oil tankers, destroyers, ferries, tugs, yachts, marine and stationary engines, motor boats capable of up to 40 knots (74kph), water tube marine boilers, etc. In addition to its London premises and its Basingstoke and Southampton works, Thornycroft had several depots and branches both at home and overseas.

Thornycroft survived financial problems during the 1930s to maintain its position as a major British manufacturing company, producing a wide range of products.

World War 2 started on 3 September 1939, at 11.15am, when British Prime Minister Neville Chamberlain announced that the UK was at war with Germany. During the six year conflict, the production capacity of engineering organisations was used to support the war effort. Among other things, Thornycroft produced parts for guns and aero engines, complete guns, exploders for bombs, depth charge throwers, as well as vehicles for military and civilian use including lorries and armoured tracked vehicles etc, and torpedo rudders.

In 1948, the company name was changed to Transport Equipment (Thornycroft) Ltd to prevent confusion with the shipbuilding Thornycroft company.

During the 1960s to the 1970s, in the popular "Dads Army" TV show, a Thornycroft van was used as Jones the butcher's van. The company was well-known for providing fire engine chassis, with multi-axle drive for uses such as airports.

They were taken over by AEC, by then Associated Commercial Vehicles Ltd and production was limited to the Nubians, Big Bens and Antars. ACV was then taken over by Leyland who already had a specialist vehicle unit in Scammell, another manufacturer of large haulage vehicles.

Thornycroft's Basingstoke factory was closed in 1969 and specialist vehicles transferred to Scammell at Watford.

Today, the Thornycroft name is used by a builder of marine diesel engines for private and light commercial use. The engines being based around small-capacity engines designed by Mitsubishi. Despite Thornycroft being effectively closed down by Leyland, the operation's parent company is now the main provider of spare parts for Leyland-built marine diesels, which for many years were highly popular for use in canal barges and narrowboats (now a market making increasing use of modern-day Thornycroft engines)