Energy supply security is a multifaceted challenge for all countries and especially for small island nations that might have limited adaptive capacity. Previous studies showed that islands experience energy scarcity and isolation from energy markets due to their remote location making energy supply security a challenging issue. We estimate energy supply diversity and concentration for 44 islands in order to provide an island specific benchmark approach for energy supply security. We use established metrics Shannon-Wiener index (SWI), Herfindahl-Hirschman index (HHI) with Energy Information Administration (EIA) fuel mix data. To confront the issues of supply security and sustainability we test energy diversity against energy and emissions intensity. The global character of the research along with the wide range of islands covered allows useful comparisons between countries and for a means of benchmarking against the indices while creating certain defined country clusters. Overall it is found that average island energy intensity increased by 23.4% with a corresponding increase of 12.4% on their emissions intensity for the period 2000–2015. On the other hand, diversity has improved by 21.3% (SWI) and by 2% (HHI) since 2000. We argue that fossil-fuel lock-in for islands must break in order to UN Sustainable Development Goal 7 to be achieved particularly for vulnerable island nations.