Sinclair qualified M.R.C.S. in 1880 and L.R.C.P. in 1881, then joined the Medical Services at the rank of Surgeon Captain on 29th July 1882. On 29th July 1894 he gained promotion to the rank of Major. In 1899 he took the Diplomas in Public Health, then served in South Africa from 1899 to 1902, during which time he was granted local rank of Colonel whilst in charge of a General Hospital, Field Force, on 6th April 1900. In 1901 he awarded the Companion Order of St Michael & St George. On 29 July 1902 he gained promotion to the rank of Lieutenant Colonel, then Colonel on 20th March 1912.
On the 19th August 1914 he embarked for France as Assistant Director of Medical Services appointed to the sea base area of Rouen. He moved in September 1914 to the sea base area at Nantes. In January 1915 he was the Assistant Director of Medical Services for the 1st Division, a post he held until 8th April 1915, when he became the Deputy Director of Medical Services of I Corps. Also in 1915 he was awarded the Companion Order of the Bath. During the Battle of Vimy Ridge the D.M.S. I Corps arranged with him to prepare a large walking wounded collecting station with No.73 Field Ambulance at Bracquemont capable of accommodating 1,500 wounded at a time, near the railway station there, and for temporary ambulance trains to run to it. On 23rd February 1916 Sinclair was retained on the Active List, then on 2nd August 1917 he became the Deputy Director of Medical Services to VI Corps. He returned to England on 2nd January 1918.