Kumar took part in Jayaprakash Narayan's movement (1974–77). He won his first election from Harnaut to the Bihar Legislative Assembly in 1985 and was elected to the Lok Sabha from Barh in 1989. In 1990 he briefly served as Union Minister of State for Agriculture.
In 1994 he co-founded the Samata Party with George Fernandes. Under Prime Minister Atal Bihari Vajpayee he served as Union Minister for Railways, Surface Transport and Agriculture (1998–2004). He resigned as minister in August 1999, taking responsibility for the Gaisal train disaster. In 2003 the Samata Party merged into the Janata Dal (United).
Kumar first became Chief Minister in March 2000, but resigned after seven days without facing a floor test. He returned to power in November 2005 with the BJP and went on to dominate Bihar politics for two decades, taking the oath of office a record ten times:
His frequent changes of alliance earned him praise for political skill as well as criticism and the nickname "Paltu Ram" from opponents.
In March 2026, Kumar filed his nomination for the Rajya Sabha and was elected unopposed. He resigned from the Bihar Legislative Council on 30 March, took oath as a Rajya Sabha MP on 10 April 2026, and resigned as Chief Minister on 15 April 2026. Samrat Choudhary of the BJP succeeded him as Bihar's first BJP Chief Minister.