Molyneux joined the International Socialists in Britain in 1968 after being radicalized by the antiwar movement and the revolt of that year. He became one of the Socialist Worker's Party's leading theorists and a popular speaker. He was a lecturer at the School of Art, Design, and Media, University of Portsmouth, from 1992.
During his years in Portsmouth, he was a significant influence in the city left as well as a lecturer at the polytechnic university there. He organized a number of demonstrations, including getting 12 coaches of people to the 2003 demonstration against the Iraq War, in London. In January 2009 he was arrested for organizing a peace rally of 400 people against the Israeli attacks on Gaza.

His book Marxism and the Party (1978), analyzes the revolutionary left approach to the political party and the question of the revolutionary organization. and the discussion of Marx, Luxemburg, Lenin, Trotsky, and Gramsci. In 1981 he published Leon Trotsky’s Theory of Revolution (1981), which critically explored Trotsky’s weaknesses and strengths. What is the Real Marxist Tradition? (1983/85) started life as a long article and was later published as a short book and is perhaps his most widely read publication.

He wrote a weekly column, “The ABCs of Marxism,” published in Socialist Worker (UK) for almost 15 years some of which were collected into a book Arguments for Revolutionary Socialism (1987) and a pamphlet on The Future Socialist Society (1987).

After he moved to Ireland he edited the Irish Marxist Review and contributed to many issues.

In 2006, he set up a blog where he "writes mainly about Marxist theory and art". He remained fascinated by this issue. His book The Point Is to Change It! was included in a display at the Tate Liverpool's exhibition Art turning Left (2013) showing the role that art plays in changing society.

In October 2020, he hosted the podcast Introduction to Marx/Marxism, which was described as "a series of short introductions to the ideas of Marx/Marxism".

In his last years, Molyneux also became involved in building an eco-socialist response to the climate crisis. He was one of the founders of the Global Ecosocialist Network.

He once wrote an article in the SWP's Internal Bulletin called "Democracy in the SWP", which argued that, though the SWP is democratic, it needs to be more so, prompting the Weekly Worker, the organ of the Communist Party of Great Britain (Provisional Central Committee) to call him a "loyal rebel". Molyneux remained a member of the loyal opposition in the SWP staying with the organization in 2012-13 when the SWP faced a major crisis in the wake of an accusation of rape against a leading member.
- Wikipedia