The threat to national security from terrorists acting on their own initiative is a challenge for law enforcement and intelligence agencies in the UK and elsewhere. The UK Parliament’s 2014 threat assessment noted ‘a trend towards ‘low signature’ terrorism by small, self-directed groups and lone actors’ [House of Commons Home Affairs Committee (2014) Counter-Terrorism – Seventeenth Report of Session 2013–14]. Lone actors have become a higher priority for counter-terrorism professionals (UK police, Prevent practitioners and security agencies), but there is a paucity of research into the views and awareness of these professionals. This qualitative study examined how Prevent practitioners perceived the radicalisation and motivations of lone-actor terrorists they had encountered. Participants were an opportunity sample of five Prevent practitioners; all had served as police officers with varying employment backgrounds and counter-terrorism experience. A thematic analysis of semi-structured interviews identified perceived general characteristics of lone-actor radicalisation. Three themes clustering around the concept of becoming a terrorist are discussed: mechanisms of radicalisation, vulnerability to radical discourse, and individual motivation. Participants construed radicalisation as a process over time, accelerated in the presence of generalised criminality or extensive Internet use. Vulnerability was seen as inherent, as well as a product of social context. Participants adopted folk-psychological explanations with mental health problems, social isolation and relative deprivation cited as prominent vulnerability factors. Lone actors were seen as motivated by grievances (e.g. deprivation), pressure from external sources (e.g. rational prospectors) or personal reward (e.g. sensation-seeking). Practitioners’ perceptions of the process over time had parallels with a diathesis–stress model, although there was some support for social movement theory.