At the very young age of seventeen, S.K. Sinha graduated with Honors from Patna University. After graduating in 1943, he joined the Indian Army. From there he passed out as the Best Cadet from the Officers' Training School, Belgaum. This honor is tantamount of the Sword of Honour at the wartime. He has also experienced the battle during the Second World War in Burma and Indonesia and then after when India became independent, in Kashmir. In Nagaland and Manipur, he also served two tenures of his career and took part in the counter insurgency operations.
The top position of the Defence Services Staff College, General Sinha obtained in 1953 at in India. He also held the same in 1962 at the Joint Services Staff College in United Kingdom. All the levels of active command in the army were held by him, which ranged from a platoon to a field army. a battalion in Ladakh, a brigade in Manipur, a mountain division in Assam, a corps in the Punjab, an infantry division in Jammu and a field Army in the Western Theatre were also commanded by him. S.K Sinha has also played an instrumental role in the staff and instructional appointments. At Army Headquarters, he served as Director, Military Intelligence, Adjutant General and Vice Chief of Army Staff. At Mhow and Staff College, Wellington, he also served as an instructor. In the year 1949, through a meeting convoked by the United Nations, he was attributed the post of Secretary of the Indian delegation on delineation of the Cease Fire Line in Kashmir.
In the year 1972, the Indian delegation to Italy for a conference on application of human rights to warfare took place and S.K. Sinha led the delegation. In the very next year 1973, the Param Vishist Sewa Medal was awarded to him. The Honorary ADC post to the President of India was conferred on him. He was also made the President of the Gorkha Brigade. The noted South Asian expert, Stephen Cohen, in a publication in the United States of America referred general Sinha as one of India's most outstanding post-independence generals.
General Sinha attempted the premature retirement from army in the year 1983. After this in the year 1990, he was appointed as the Ambassador of India to Nepal. At that time dictatorial rule was prevailed in that country. And in the wake of the trade and transit situation of 1989, this isobilateral relation with India had hit their celestial point. Democracy was regenerated in Nepal during the tenureship of S.K.Sinha and the relations between India- Nepal were raised to a high level of cordial disposition. The Prime Minister of India acknowledged his contribution by stating that he had played a major role in this development. The Prime Minister of Nepal wrote, "General Sinha was as much India's Ambassador to Nepal as Nepal's Ambassador to India".
By the year 1997, the insurgency took its peak in Assam. In that juncture General Sinha was made the Governor of Assam. A three pronged counter-insurgency strategy was crafted by him. He also managed to fetch the hearts and minds of people of Assam. As a result and through an attitudinal alteration among the people of Assam, the state emerged out of the quagmire of militancy. He received countless letters saying that he was more Assamese than any Assamese and he was a true son of the soil of Assam, on his departure from Assam.
General Sinha was sworn in as the 11th Governor of the state of Jammu and Kashmir, On 4th June, 2003. He continues to serve in that capacity. Apart from his army career and political career, General S.K Sinha has also been a prolific writer. He has contributed nearly 300 edit page articles in national newspapers. He has authored five books on his credit, including one on Jammu and Kashmir Operation of 1947-48 and his autobiography, "A Soldier Recalls" and rests are "Of Matters Military", "Pataliputra" and "Veer Kuer Singh".