This article provides a comparative analysis of lone-actor terrorist radicalization from a relational perspective. Extant research on lone-actor terrorism has shown that lone actors are rarely as “lone” as public perceptions suggest. In most cases, lone-actor terrorists have some social ties to established radical groups. Accordingly, this article asks (1) why these individuals do not integrate into the radical groups they frequent and engage in collective violence, and (2) if they do integrate, why do they then end up engaging in violence on their own? The article argues that patterns of lone-actor terrorist radicalization can be categorized according to the extent and evolution of their loneness. It highlights two broad patterns of lone-actor radicalization in relation to broader radical groups/movements–peripheral and embedded–and explores the reasons why some lone-actor terrorists remain peripherally integrated in radical groups, while others become more embedded only to engage in violence alone. The article is based on qualitative research, drawing on a geographically and ideologically diverse sample of cases (N = 25), and access to restricted material. The article identifies and theorizes five recurrent radicalization trajectories, which are variations of the peripheral and embedded patterns, and discuss the implications for prevention/interdiction.