Duncan Hallas was a prominent member of the Trotskyist movement in Britain. He was first a member of the Workers International League in the early 1940s, and then its successor organisation the Revolutionary Communist Party after World War II. When factional disputes broke out in The Club, the name adopted by the British Trotskyists after entering the Labour Party, Hallas became a supporter of Tony Cliff's positions.

He was then a founder member of the tiny Socialist Review Group when it was organised in 1951. As a member of the SRG he wrote its only major founding document not authored by Cliff entitled On the Stalinist Parties. He also wrote a number of articles for the early issues of Socialist Review but when his job took him to Scotland in 1954/5 he seems to have dropped out of the group.

With the upsurge in struggles in 1968 Hallas joined the International Socialists (IS) and rapidly became a member of the groups leadership. As such he was a full time worker for the group and daily involved in the struggles of those years.

Like many of the senior cadre of the IS Hallas became concerned by the role played within the leadership by Tony Cliff and his increasing tendency to take decisions without consulting leadership bodies. This reached such a point that Hallas initiated an oppositional grouping to Cliff's course in the IS alongside John Palmer and Jim Higgins. However when this oppositional grouping became formalised as the International Socialist Opposition (ISO), a formal faction, Hallas broke with it. The result was the defeat of the ISO and the expulsion of its members.

Hallas would continue to be a member of the leadership of the IS and then of the Socialist Workers Party, its organisational successor founded in 1977, until his retirement from active politics in the late 1990s.

He was the author of inumerable articles for the socialist press and a short guide to the politics of Leon Trotsky.