The infamous Peterloo Massacre long provided a foothold for the political left to call for further democratic rights in Britain. However, the meaning of Peterloo in 1819 and its legacy in the century that followed were not one and the same. Parliamentary reforms during the Victorian period forced the political left to reorganize the meaning of Peterloo for the event to remain relevant. Anchored by both socialists and Irish Home Rule supporters, Peterloo’s primary association with the right of public assembly came to the fore during the 1880s. Then, after the First World War, the memory of Peterloo again was rebranded by leftist groups in a Marxist interpretation. In both instances, those invoking the 1819 event hoped to adopt those at Peterloo as their spiritual predecessors. This article aims to demonstrate the versatile nature of the memory of Peterloo for the political left. The rhetoric surrounding Peterloo commanded the event in different ways in different political environments, and indeed through different mediums. When viewed over the course of Britain’s century of reform, the evolving representations and invocations of the Peterloo Massacre demonstrate the incredible malleability and endurance of the 1819 event.