This article examines the experience of English working-class radicals imprisoned on charges of treason and sedition in the early nineteenth century. It offers a new analysis of the impact of government repression of the parliamentary reform and trades movements, and the involvement of reformist Whig MPs in their cause. It analyzes previously unstudied collections of state prisoners’ letters confiscated by gaolers and sent to the Home Office, and their petitions to parliament for gaol reform and against the Suspension of Habeas Corpus Act. It argues that though the radicals faced more severe conditions than their middle-class London counterparts, the experience of attempting political activity within prison shaped the constitutionalist nature of working-class radicalism. The radicals connected their grievances with conditions in county gaols ‘reformed’ under disciplinary regimes with larger government corruption and oppression under the Suspension of Habeas Corpus Act. Their complaints formed part of the wider prison reform movement that led to Robert Peel’s Gaols Acts of 1823 and 1824. This article is a major contribution to understanding the wider context and definitions of reform in the early nineteenth century.