Gen Sinha was promoted to captain on 10 September 1951. In 1953, Sinha secured the top position at the Defence Services Staff College in India and in 1962 again, at the Joint Services Staff College in the United Kingdom. He held all levels of active command in the Army from a platoon to a field army. He was promoted lieutenant-colonel on 9 June 1965. He commanded a battalion in Ladakh, a brigade in Manipur, a mountain division in Assam, an infantry division in Jammu, a corps in Punjab, and the Western Army. He held key staff and instructional appointments. He served as Director, Military Intelligence, Adjutant General, and Vice Chief of Army Staff at Army Headquarters. He also served as an instructor at Mhow and Staff College, Wellington. During his Army career, he was associated with Jammu and Kashmir from Day One that is, 27 October 1947. He was involved as a junior staff officer in organizing the massive airlift from Delhi to Srinagar in October 1947, in the wake of the attack from across the border in Pakistan, which was critical to the Indian efforts to beat back the raiders. In 1949, he was appointed Secretary of the Indian delegation on the delineation of the Cease Fire Line in Kashmir at a meeting convened by the United Nations. He led the Indian delegation to Italy in 1972 for a conference on the application of human rights to warfare. He was awarded the Param Vishisht Seva Medal in 1973 and was promoted to major-general on 28 December. He was made Honorary ADC to the President of India. He also served as the President of the Gorkha Brigade.