In March 1911 at the age of 13, he succeeded to the Jodhpur gadi upon the death of his father, and served as a page of honour to George V at the Delhi Durbar that year.

Upon the outbreak of the First World War, the young Maharaja volunteered to serve in combat and was commissioned as an honorary Lieutenant in the British Army in October 1914.

Shortly after, he left India for the battlefields of the Western Front, leading the Jodhpur Imperial Service Lancers in combat in France and Flanders.

[3] On 9 December 1915, Sumer Singh married Pratapha Kanwarji Maji Sahiba, the daughter of Kumar Jivansinhji Jhalamsinhji Sahib of Sarodar, a branch of the Nawanagar royal family, and the sister of cricketer Ranjitsinhji.

[4] He married secondly, on 23 May 1918, Umrao Kanwarji Sahiba (d. 30 November 1949), the daughter of Thakur Shri Suraj Malji, of Sointra, under Jodhpur Marwar.