Relatively little effort has been made to develop and validate theories that explain firesetting. In this study, the first offense chain model of firesetting in mentally disordered offenders was developed. Twenty-three mentally disordered firesetters were interviewed about the affective, cognitive, behavioral, and contextual factors leading up to and surrounding one of their recorded firesetting offenses. Offense account interviews were analyzed using grounded theory. The resulting model consists of four main phases: (a) background, (b) early adulthood, (c) pre-offense period, and (d) offense and post-offense period. The model accounts for firesetting by male and female mentally disordered offenders and highlights the importance of early childhood experiences of fire and the onset of mental illness as precursors to firesetting within this population. Furthermore, the model is able to distinguish between different types of mentally disordered firesetters and their offense styles. The clinical implications and utility of the model are also discussed.