Born in India into a Communist family in the Punjab, Bains became a member of the youth wing of the Communist Party of India (CPI). He was dismayed by what he saw as the revisionism of Nikita Khrushchev following the death of Joseph Stalin, and he broke with the CPI when it supported Khrushchev's criticisms of Stalin.
In 1963, he founded the Internationalists, an anti-revisionist organization that supported Mao Zedong's Communist Party of China in the Sino-Soviet split. This organisation (in Canada) ultimately became the Communist Party of Canada (Marxist-Leninist) (CPC-ML), of which Bains was the founding leader.
In 1967, Bains held a conference in London to determine the future of the anti-revisionist movement, the "Necessity for Change" conference. The Irish Communist Organisation disagreed with the other delegates and walked out of the meeting. Bains was a leader of the anti-revisionist movement internationally, and assisted in establishing Marxist-Leninist parties around the world, such as the Revolutionary Communist Party of Britain (Marxist-Leninist), the Communist Party of Trinidad and Tobago, the Communist Party of Ireland (Marxist-Leninist), and the Communist Ghadar Party of India. Bains was also responsible for the founding of the Hindustani Ghadar Party (Organisation of Indian Marxist-Leninists Abroad). He held a leading influence in the Marxist-Leninist Party, USA in the 1970s. (This party was dissolved in 1993.) Modern Communism has written articles on his Marxist-Leninist legacy.
Many of these parties originally held a "Maoist" position in the 1960s and 1970s, and Bains was often identified as a Maoist. However, following Mao Zedong's death in 1976, they would later reject Mao Zedong Thought, particularly through the experience of the Sino-Albanian split. Following the leadership of Enver Hoxha and the Albanian Party of Labour, Bains' anti-revisionism deepened in the sense of opposition both to the European revisionism (Khrushchev, Josip Broz Tito and Eurocommunism) and to the Chinese revisionism.
A memorial was erected in the honour of Bains and other CPC-ML "fallen comrades" in Ottawa's Beechwood Cemetery. Poet George Elliot Clarke published a poem titled "Homage to Hardial Bains" in 2000 in the Oyster Boy Review. Bains was attacked posthumously by Ben Seattle in an article on Leninism.org entitled In memory of a charlatan.
Bains wrote several books, including Necessity for Change!, Modern Communism, Visiting Cuba, If You Love Your Class and Thinking About the Sixties, as well as many more articles, pamphlets and speeches.
Sandra L. Smith, his widow, also served as leader of the CPC-ML.