Sushil Kumar was born in a HinduJat family of Baprola village, near Najafgarh in South West Delhi. His father, Diwan Singh, was a DTC bus driver, while his mother, Kamla Devi, is a housewife. He was inspired to take up wrestling by his cousin Sandeep and his father, who wrestled himself. Sandeep later stopped competing 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 devoted Hindu and strict 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. He was trained at the akhada by Yashvir and Ramphal, later by Arjuna awardee Satpal and then at the Indian Railways camp by Gyan Singh and Rajkumar Baisla Gurjar.
His first success came at the World Cadet Games in 1998 where he won the gold medal in his weight category, followed by a gold at the Asian Junior Wrestling Championship in 2000. Moving out of the junior competitions, in 2003 Kumar won the bronze medal at the Asian Wrestling Championships and a gold at the Commonwealth Wrestling Championships. Kumar placed fourth at the 2003 World Championships, but this went largely unnoticed by the Indian media as he fared badly in the 2004 Olympic Games in Athens, placing 14th in the 60 kg weight class. He won gold medals at the Commonwealth wrestling championships in 2005 and 2007. He placed seventh in the 2007 World Championships and won a bronze medal at the 2008 Olympic Games in Beijing. He won a silver medal at the 2012 Summer Olympics, becoming the first person to win two Olympic medals for independent India. Kumar was awarded the Arjuna Award in 2006 and a coveted Padma shree by the Indian Government in 2011.
Kumar lost to Andriy Stadnik in the first round of the 66 kg freestyle wrestling event, leaving his medal hopes hinging on the repechage. He defeated Doug Schwab in the first repechage round and Albert Batyrov in the second round. In the bronze medal match on 20 August 2008, Kumar beat Leonid Spiridonov 3:1. 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.
At the 2010 World Wrestling Championships Kumar became the first Indian to win a world title in wrestling. He beat the local favourite Alan Gogaev of Russia 3–1 in the finals in the 66 kg category. Earlier, he had won a close semi-final match against European champion Jabrail Hasanov of Azerbaijan 4–3 to make it to the final (he was losing 2–3 and scored a 2-point move in the last 5 seconds).
Kumar won gold medal at the 2010 Commonwealth Games held in Delhi on 10 October 2010. He beat Heinrich Barnes 7–0 in the finals in the 66 kg freestyle wrestling category. The bout was stopped by the referee in the second round. Earlier, in the semifinals, Kumar defeated Famara Jarjou 3–0 within 9 seconds. In the quarterfinals, Kumar defeated Muhammad Salman 10–0 in 46 seconds.