During the First World War Thornycroft developed the first coastal motor torpedo boats and launching systems for depth charges and was knighted for his work.

[5] He was educated at St Paul's School, London before receiving engineering training at the Central Technical College in South Kensington.

[2] Completing his training in 1895 Thornycroft was appointed a draftsman and assistant to the manager of the engine works at the Chiswick yard, where he helped install machinery to torpedo boats and destroyers.

[2] The family business was converted into a publicly listed company in 1901 and Thornycroft was appointed technical director in 1902, with responsibility for all construction work and new designs, including motor vehicles.

[2] Thornycroft worked on the design and construction of vessels for the Royal Navy during the First World War, a service for which he was appointed a Knight Commander of the Order of the British Empire in 1918.