Singh was born on 3 January 1938 at village Jasol, Barmer district, Rajasthan in a Rajput family. His father was Thakur Sardara Singh Rathore and mother was Kunwar Baisa. Singh is married to Sheetal Kanwar. They have two sons. his elder son Manvendra Singh former Member of Parliament from Barmer. He also has two grandchildren (children of Manvendra singh Rathore) Harshini Kanwar Rathore (Granddaughter) and Hamir Singh Rathore (Grandson). He was an officer in the Indian Army in the 1960s and is an alumnus of Mayo College and the National Defence Academy (India), Khadakwasla.

Though Singh entered politics in the late 60s, the first few years of his political life lacked success and recognition, till he was initiated in the Jan Sangh, by BJP stalwart Bhairon Singh Shekhawat who is considered to be Jaswant's political mentor. He tasted success in his political career in 1980 when he was first selected for the Rajyasabha, the upper house of Indian parliament. He served as Finance minister in the short-lived government of Atal Bihari Vajpayee, which lasted just from 16 May 1996, to 1 June 1996. After Vajpayee became Prime Minister again two years later, he became Minister for External Affairs of India, serving from 5 December 1998 until 1 July 2002. Responsible for foreign policy, he dealt with high tensions between India and Pakistan. In July 2002 he became Finance Minister again, switching posts with Yashwant Sinha. He served as Finance Minister until the defeat of the Vajpayee government in May 2004 and was instrumental in defining and pushing through the market-friendly reforms of the government. He was conferred the Outstanding Parliamentarian Award for the year 2001. On 19 August 2009, he was expelled from BJP after criticism over his remarks in his book which allegedly praised the founder of Pakistan in his book Jinnah – India, Partition, Independence. His last major position was as Leader of Opposition in the Rajya Sabha from 2004 to 2009.

Singh is widely regarded for his handling of relations with the United States which were strained after the 1998 Indian nuclear tests but which ameliorated soon after culminating in the 2000 visit of U.S. President Bill Clinton to India. His skill as a negotiator and diplomat during talks with the United States has been well acknowledged by his U.S. counterpart Strobe Talbott.

Controversy hovered around him again when on 17 August 2009 another book authored by him, entitled Jinnah: India-Partition-Independence, was released. In this he praised Mohammad Ali Jinnah and claimed that the centralised policy of Jawaharlal Nehru was responsible for Partition. He was later expelled from the primary membership of BJP as a result of the ensuing controversy. In interviews with media he quoted BJP as narrow-minded and to have limited thought. In 2010, he was readmitted to BJP.

Shri Jaswant Singh was elected from Darjeeling Seat from the year 2010 to 2014. His contribution to the hills and its people is evident for he touched the lives of so may organisations and advised the political movement with all his experience and foresight.

He was the candidate for the post of Vice President for the NDA in 2012. On 6 August 2012, Tamil Nadu Chief Minister Jayalalithaa extended AIADMK’s support to NDA's vice-presidential nominee Jaswant Singh saying that there has to be an opposition in a true democracy. He lost to Hamid Ansari who was the UPA's Vice presidential candidate.

Singh was denied a Member of Parliament ticket for Barmer by BJP for the Indian general election, 2014 over Col. Sonaram Choudhary. Unhappy, Singh filed his nomination as an independent candidate from Barmer constituency. Subsequently, he was expelled from BJP for six years and lost the election.