Mentalising, also known as "Theory of Mind", is the ability to understand and infer the cognitions of others, such as their perceptions, intentions, and beliefs. Although several tools have been designed to measure mentalising in adults, there exist methodological and practical limitations. Many of the existing measures conflate mentalising with similar constructs (e.g., empathy), and most are lengthy measures that are unsuitable for large population-based studies and clinical practice. These issues are currently hampering clinical and non-clinical investigations into mentalising and related social-cognitive abilities. Drawing on questionnaire measures of social cognition, we conceived a self-report mentalising scale, the Four-Item Mentalising Index (FIMI; Studies 1a and b). The FIMI was developed through a series of studies examining its factor structure and reliability (Studies 2a and b) and by testing its construct validity against a cognitive mentalising task, autistic traits, and comparing scores in autistic and non-autistic people (Studies 3a and b). Together, we demonstrate that the FIMI is a conceptually and methodologically robust tool for measuring mentalising ability in the general population, including autistic and non-autistic people. Future research directions and practical (clinical) applications of the scale are discussed, with a focus on improving understanding and management of (a)typical mentalising ability. (PsycInfo Database Record (c) 2021 APA, all rights reserved).