Difficulties with executive functions (EF) are very common among individuals with Williams syndrome (WS). To characterise the pattern of relative strengths and weaknesses in EF for children and adolescents with WS, we considered the performance of a large sample on the parent version of the Behavior Rating Inventory of Executive Function-2 (BRIEF-2). Associations between distinct components of EF and adaptive behaviour, behaviour problems and intellectual ability were investigated. The concurrent effects of components of behaviour regulation and emotion regulation on attention problems and anxiety problems also were evaluated. Participants were 308 6-17-year-olds with genetically confirmed classic WS deletions. Parent report of EF was measured by the BRIEF-2 questionnaire. Most participants (223/308) completed the Differential Ability Scales-II as a measure of intellectual ability. The parents of these individuals also completed the Child Behavior Checklist and the interview form of the Scales of Independent Behavior-Revised. As a group, the participants evidenced considerable parent-reported EF difficulty. A profile of relative strength and weakness was found at the index level, with performance on both the Behavior Regulation Index and the Emotion Regulation Index significantly better than performance on the Cognitive Regulation Index. Within each index, a statistically significant pattern of relative strength and weakness also was identified. Difficulties with behaviour regulation and emotion regulation were related to both behaviour problems and adaptive behaviour limitations. Higher inflexibility and more difficulty with self-monitoring were associated with lower overall intellectual ability. Difficulty with inhibition was uniquely associated with attention problems, and inflexibility was uniquely associated with anxiety problems. Executive function difficulties are highly prevalent among children and adolescents with WS and are associated with adaptive behaviour limitations, both internalising and externalising behaviour problems and more limited intellectual ability. These results highlight the importance of designing and delivering research-based interventions to improve the EF of children and adolescents with WS.