Accoring to Marshall Robert Sinclair was born in London on 1 July 1817, but according to Ellis came from a Caithness family, and was eduacated at Charterhouse. He was apprenticed to shipbuilders and later served with Robert Stephenson, the Grand Junction Railway, and the Paris & Rouen Railway. The last two positions initiated him in the traditions of Allan and Buddicom.
When he became the first locomotive superintendent of the Caledonian Railway in 1848, at the age of thirty-two, his locomotive designs were in the Allan style. On the Eastern Counties Railway (later Great Eastern), which he joined in 1856, he continued this line, but also introduced a completely new design of 2-4-0 passenger engine with stovepipe chimney. He provided cabs of unprecedented protectiveness, against which his enginemen strongly protested. As consultant engineer to the Great Luxemburg Railway he sponsored the introduction of a fast 2-4-2 tender locomotive with Bissell trucks, a type that became popular on mainland Europe.
Later Sinclair adapted the concept as a 2-4-2 tank locomotive for the Great Eastern's commuter services. As consulting engineer for the East Indian Railway, he designed an outside cylinder 2-4-0 which was built by W.G. Armstrong & Co. Whilst with the GER a Sinclair bogie single was painted "in a sort of virginal cream colour" for the wedding journey of the Prince and Princess of Wales to Wolferton on 10 March 1863. Between 1857, following the retirement of Bruff and 1866 Sinclair also acted as Civil Engineer to the GER. He was an early exponent of standardization, the Giffard injector and employed steel for axles and tyres earlier than most, and used roller bearings for carriages well ahead of their time.He retired to Italy where he did not die until 20 October 1898 and as Ellis notes "was becoming more and more forgotten in the land of his birth".
John Thomas's Springburn story includes: Sinclair was old and ill and living quietly in retirement far away in Florence [at the time of the 1896 Caledonian Railway Springburn soirée]. When he died two years later Mr C. Du Riche Preller, writing in one of the professional journals, gave this picture of Sinclair in his last days: 'He lived,' wrote Preller, 'chiefly for and in his beautiful and extensive library, ever fresh in intellect and active in body, although well advanced in years. Sincerely beloved and respected by all who served under him professionally he was equally so by all who knew him in his private life, or like myself, in his placid, cultural retirement where few were aware that the Mr. Sinclair of refined literary tastes was Sinclair the engineer of the Caledonian and Great Eastern Railway fame.'
See: H. Ellis, Twenty Locomotive Men (1958).