This article explores the role of converts in the ‘Global Salafi Jihad’ based on a sample of 75 American and 47 British converts who became involved in Islamist terrorism between 1980 and September 11th, 2013. Converts are compared to non-coverts on a variety of demographic, operational and investigative variables, and each sample is further divided into those who mobilized before and after 9/11 in order to allow assessment of changes over time. Contrary to previous research, results show that American converts in particular constitute a “jihadi underclass” that is markedly disadvantaged compared to the rest of the U.S. sample. They also present a generally less capable terrorism threat and are especially likely to be caught in sting operations. British converts, whilst also clearly disadvantaged and less capable, are much less distinct compared to non-converts in the U.K. Practical and theoretical implications of these findings are discussed.