The prevalence of criminal backgrounds amongst European jihadists is remarkable. Whether amongst ‘foreign fighters’ that have travelled to Syria and Iraq, or amongst those involved in terrorism in Europe, criminal pasts are common. Yet, they remain unexamined. This article presents a unique empirical examination of 79 European jihadists with criminal backgrounds, examining the relevancy of their criminal pasts in relation to their terrorist futures. The results fall into four themes. Firstly, jihadism can affect a criminal’s radicalisation process in two ways: it can offer redemption from past sins, or it can legitimise crime. Secondly, prisons offer an environment for radicalisation and networking amongst criminals and extremists. Thirdly, criminals develop skills that can be useful for them as extremists, such as access to weapons and forged documents, as well as the psychological ‘skill’ of familiarity with violence. Finally, white-collar and petty crime is often used to finance extremism. The results challenge conceptions on radicalisation, and can affect counter-terrorism responses.