Kumar was born into the Jat family of Shivdan Singh and lived in the village of Jhalara, Tehsil Neem-Ka-Thana, in the Sikar District in Rajasthan, India. His mother's name was Rajkaur. Shivdan Singh was a strong follower of Arya Samaj, who joined the Indian army and was badly wounded in Indo-Pakistani War of 1947.:21 Kumar has a daughter (Samita) and two sons (Japender and Mahaveer). Mahaveer studied in Sanskar School Jaipur and is a national basketball player. Japender studies in Delhi Technological University (formerly DCE). Samita is a national Kabaddi player.
Kumar joined the 2nd Battalion of the Rajputana Rifles on 3 September 1985. After training was complete, his battalion was posted to Kashmir. In 1987 he was selected for the Indian Peace Keeping Force and took part in Operation Pawan in Sri Lanka.
Operation Pawan was the code name assigned to operations by the Indian Peace Keeping Force to take control of Jaffna from the LTTE in late 1987, enforcing the disarmament of the LTTE as a part of the Indo-Sri Lankan Accord. After fighting that lasted for about three weeks, the IPKF took control of the Jaffna Peninsula from the LTTE, something that the Sri Lankan army had attempted and failed to achieve for several years. Supported by Indian army tanks, helicopter gunships, and heavy artillery, the IPKF routed the LTTE, although they lost 214 soldiers in the process.
Kumar and his group were assigned to patrol dominant areas in Tamil. Five Tamil militants fired and killed five soldiers from Kumar's squad. In return, Kumar and the remaining soldiers of his squad followed the militants into the house of an MLA. However, the MLA opposed the action, and in the ensuing firefight the MLA, along with the five Tamil militants, was killed. This caused some controversy, and Kumar was penalised and remanded.:31
Meanwhile, 36 soldiers of the 10th Parachute Regiment were captured and held by the LTTE. Lt. Gen. A.S. Kalkat assigned the task of freeing them to Kumar. Kumar took with him 50 kg ammunition and some biscuits and sailed up the river. Kumar saved the soldiers (who had been held in the forest for 72 hours), destroyed an important ammunition depot, and killed 39 militants.:43