Sushil Kumar was born in a Jat family in the village of Baprola, near Najafgarh in South West Delhi. Kumar's father is Diwan Singh, a Jat, who was a DTC bus driver and mother Kamla Devi a housewife. He was inspired to take up wrestling by his cousin Sandeep and his father who was himself a pehlwan (wrestler). Sandeep quit wrestling as the family could only support one wrestler. Kumar trained at the akhada (wrestling school) in the Chhatrasal Stadium from the age of 14. With minimal funds and poor training facilities for wrestling in India, even for the 2008 Olympic team, his family made sure he obtained the necessary dietary supplements by sending him tinned milk, ghee and vegetables. He is a vegetarian. Kumar is presently employed with the Indian Railways as an Assistant Commercial Manager.
Kumar started training at the Chhatrasal Stadium's akhada at the age of 14. Trained at the akhada by Indian pehlwans Yashvir and Ramphal, and later by Arjuna awardee Satpal and then at the Railways camp by coach Gyan Singh, Sushil endured tough training conditions which included sharing a mattress with a fellow wrestler and sharing a dormitory with twenty others. at the age of 18 he became state champ. His first success came at the World Cadet Games in 1998 where he won the gold medal in his weight category. He followed this up with a gold in the Asian Junior Wrestling Championship in 2000. Moving out of the junior competition, Sushil Kumar won the bronze medal at the Asian Wrestling Championships in 2003 and followed that up with a gold medal at the Commonwealth Wrestling Championships. Sushil Kumar placed fourth in the World Championships in 2003, but this went largely unnoticed by the Indian media as he fared badly in the 2004 Olympic Games in Athens, in the 60 kg class placing 14th. He won gold medals at the Commonwealth Wrestling Championships in 2005 and 2007. He ranked seventh in the 2007 World Wrestling Championships and won a bronze medal in 2008 Olympic Games in Beijing. He also qualified for 2012 Summer Olympics taking place at London and won a silver medal by defeating a wrestler from Kazakhstan, thus becoming the first person to win 2 Olympic medals for independent India. Sushil Kumar was awarded the Arjuna Award in 2006.
Out of the field of 21, 11 wrestlers including Kumar obtained a bye to the 1/8 round. He lost to Andriy Stadnik from Ukraine in the first round of the 66 kg freestyle wrestling event, leaving his medal hopes hinging on the repechage. Sushil Kumar defeated American Doug Schwab in the first repechage round and Belarusian Albert Batyrov in the second repechage round. In the bronze medal match on 20 August 2008 Kumar beat Spiridonov 3:1, with scores of 2-1, 0-1, 2-0 in the three rounds. Sushil Kumar disclosed that he had no masseur during the three bouts he won within a span of 70 minutes to take the bronze. The team manager Kartar Singh who is a former Asian Games medallist acted as the masseur for him.
Sushil Kumar created history when he became the first Indian to win a gold medal at FILA 2010 World Wrestling Championships held in Moscow on 12 September 2010. Sushil Kumar beat local hope & crowd favourite Alan Gogaev of Russia 3-1 in the finals for the gold in the 66 kg freestyle wrestling category. Earlier, he had won a thrilling semi-final match against European champion Jabrail Hasanov of Azerbaijan 4-3 to make it to the final (He scored a 2-point move in the last 5 seconds to come from 2-3 behind). After getting a bye in the first round, Sushil had routed Akritidis Anastasios of Greece 6-0 in the second round, beat Martin Sebastian of Germany 4-1 in the pre-quarters & demolished Mongolian Buyanjav Batzorig 9-1 in the quarter-finals.
Sushil Kumar won gold medal at the 2010 Commonwealth Games held in Delhi on 10 October 2010. He beat Heinrich Barnes of South Africa 7-0 in the finals in the 66 kg freestyle wrestling category. The South African wrestler could not endure more than three and half minutes in front of Sushil. In fact the bout had to be stopped by the referee in the second round. Earlier, in the semifinals, Sushil defeated Famara Jarjou of the Gambia 3-0 in a record 9 seconds. In the quarterfinals Sushil Kumar defeated Muhammad Salman of Pakistan 10-0 in 46 seconds.
Sushil Kumar won the Silver medal of 66 kg Free Style Wrestling after losing the final to Tatsuhiro Yonemitsu of Japan. Earlier he had entered the final amid some controversy by beating Kazakhastan's Akzhurek Tanatarov in the semifinal. The Kazakh athlete claimed Kumar had bitten his ear Sushil had also led the Indian contingent at the 2012 London Summer Olympic Games and was India's flag bearer for the London Olympics opening ceremony on 27 July.