After graduating from Panjab University, Thapar earned her doctorate under A. L. Basham at the School of Oriental and African Studies, the University of London in 1958. She was a reader in Ancient Indian History at Kurukshetra University between 1961 and 1962 and held the same position at Delhi University between 1963 and 1970. Later, she worked as Professor of Ancient Indian History at the Jawaharlal Nehru University, New Delhi, where she is now Professor Emerita.
Thapar's major works are Aśoka and the Decline of the Mauryas, Ancient Indian Social History: Some Interpretations, Recent Perspectives of Early Indian History (editor), A History of India Volume One, and Early India: From the Origins to AD 1300.
Her historical work portrays the origins of Hinduism as an evolving interplay between social forces. Her recent work on Somnath examines the evolution of the historiographies about the legendary Gujarat temple.
In her first work, Aśoka and the Decline of the Maurya published in 1961, Thapar situates Ashoka's policy of dhamma in its social and political context, as a non-sectarian civic ethic intended to hold together an empire of diverse ethnicities and cultures. She attributes the decline of the Mauryan empire to its highly centralised administration which called for rulers of exceptional abilities to function well.
Thapar's first volume of A History of India is written for a popular audience and encompasses the period from its early history to the arrival of Europeans in the sixteenth century.
Ancient Indian Social History deals with the period from early times to the end of the first millennium, includes a comparative study of Hindu and Buddhist socio-religious systems, and examines the role of Buddhism in social protest and social mobility in the caste system. From Lineage to State analyses the formation of states in the middle Ganga valley in the first millennium BC, tracing the process to a change, driven by the use of iron and plough agriculture, from a pastoral and mobile lineage-based society to one of settled peasant holdings, accumulation and increased urbanisation.
Thapar has been a visiting professor at Cornell University, the University of Pennsylvania, and the College de France in Paris. She was elected General President of the Indian History Congress in 1983 and a Corresponding Fellow of the British Academy in 1999.
She was awarded the prestigious Jawaharlal Nehru Fellowship in 1976. Thapar is an Honorary Fellow at Lady Margaret Hall, Oxford, and at the School of Oriental and African Studies (SOAS), University of London. She holds honorary doctorates from the University of Chicago, the Institut National des Langues et Civilisations Orientales in Paris, the University of Oxford, the University of Edinburgh (2004), the University of Calcutta (2002) and recently (in 2009) from the University of Hyderabad. She was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 2009.
In 2004, the US Library of Congress appointed her as the first holder of the Kluge Chair in Countries and Cultures of the South.
In January 2005, she declined the Padma Bhushan awarded by the Indian Government. In a letter to President A P J Abdul Kalam, she said she was "astonished to see her name in the list of awardees because three months ago when I was contacted by the HRD ministry and asked if I would accept an award, I made my position very clear and explained my reason for declining it". Thapar had declined the Padma Bhushan on an earlier occasion, in 1992. To the President, she explained the reason for turning down the award thus: "I only accept awards from academic institutions or those associated with my professional work, and not state awards".
She is co-winner with Peter Brown of the prestigious Kluge Prize for the Study of Humanity for 2008 which comes with a US$1 million prize.
Thapar is critical of what she calls a "communal interpretation" of Indian history, in which events in the last thousand years are interpreted solely in terms of a notional continual conflict between monolithic Hindu and Muslim communities. Thapar says this communal history is "extremely selective" in choosing facts, "deliberately partisan" in interpretation and does not follow current methods of analysis using multiple, prioritised causes.
In 2002, the Indian coalition government led by the Bharatiya Janata Party (BJP) changed the school textbooks for social sciences and history, on the ground that certain passages offended the sensibilities of some religious and caste groups. Romila Thapar, who was the author of the textbook on Ancient India for class VI, objected to the changes made without her permission that, for example, deleted passages on eating of beef in ancient times, and the formulation of the caste system. She questioned whether the changes were an, "attempt to replace mainstream history with a Hindutva version of history", with the view to use the resultant controversy as "election propaganda." Other historians and commentators, including Bipan Chandra, Sumit Sarkar, Irfan Habib, R.S. Sharma, Vir Sanghvi, Dileep Padgaonkar and Amartya Sen also protested the changes and published their objections in a compilation titled, Communalisation of Education.
Writing about the 2006 Californian Hindu textbook controversy, Thapar opposed some of the changes that were proposed by Hindu groups to the coverage of Hinduism and Indian history in school textbooks. She contended that while Hindus have a legitimate right to a fair and culturally sensitive representation, some of the proposed changes included material that pushed political agenda.
Thapar's appointment to the Library of Congress's Kluge Chair in 2003 was opposed in an online petition bearing more than 2,000 signatures, on the grounds that she was a "Marxist and anti-Hindu" and it was "waste of US money" to support a leftist. Journalist Praful Bidwai criticised the petition as a "vicious attack" by communalists who are "not even minimally acquainted" with her work. A number of academics sent a protest letter to the Library of Congress denouncing the petition as an attack on intellectual and artistic freedom. The Communist Party of India (Marxist) supported her appointment by calling her "a liberal with a scientific outlook". Martha Nussbaum has stated that Thapar is neither a communist nor a Marxist historian and the Library of Congress treated the petition with "the indifference that it deserved".