Born Berthe Dutoit in the Valais area of French Switzerland, the daughter of a socialist engineer, Hamilton moved to Paris as a young woman.
Hamilton was for many years closely linked with Gerry Healy but, in 1971, she sided with Pierre Lambert and the French OCI Internationalist Communist Organisation when the ICFI split into two, ostensibly over the place of philosophy in Marxist theory, but equally about control of the ICFI after the OCI grew to be larger than the SLL in the late 1960s.
She took public stands as a Trotskyist, condemning Healy's allegations that Joseph Hansen, one of the American Socialist Workers Party leaders, was a CIA agent.
[8] Hamilton co-founded the British Committee for the Reconstruction of the Fourth International (BCRFI) after disagreements with Robin Blick and John Archer within the Bulletin Group, which then split.
She maintained an active membership of the Labour Party for most of her life in Britain, in North London, in Newark during the war and, from the 1940s until old age, in Westminster.