He was involved in the Vietnam Solidarity Campaign and outraged many leftists when, at a meeting in the Conway Hall, he denounced Ho Chi Minh for murdering the leader of the Vietnamese Trotskyist movement, Tạ Thu Thâu, in 1945 after crushing the workers' rising of that year in Saigon.

His main role in the IS (from 1977 the Socialist Workers Party, or SWP) was as a theorist and he produced numerous books and articles on a wide variety of topics.

He was first editor of Socialist Worker in 1976-77 and returned to the role after a break in 1982, remaining in the post until 2004,[2] when he started editing the SWP's theoretical quarterly International Socialism Journal.

Harman's work on the May 1968 events in France and other student and workers' uprisings of the late 1960s, The Fire Last Time, was recommended by rock band Rage Against the Machine in the album liner notes for Evil Empire.

He is buried in Highgate Cemetery, London, a few yards from Karl Marx's tomb and adjacent to his comrade Paul Foot.