Objectives: Work performance is a critical aspect of daily living, significantly impacted by the characteristics of ADHD. However, current research lacks sophisticated, theoretically, and empirically supported instruments for assessing work performance in this context. Therefore, this study aimed to develop a comprehensive and psychometrically sound self-report instrument for assessing the work performance of adults with ADHD and related disorders. Methods: An initial set of 40 work performance items was developed based on a multifactorial model, various preexisting questionnaires, and input from an interdisciplinary working group. Extensive psychometric analyses were conducted on a community sample of 714 participants engaged in paid work for at least 9 hr a week, randomly split into two subsamples. Results: Confirmatory factor analyses (CFA) validated the 7-factor structure of the work performance scale. The initial set of items was reduced by 10 based on internal consistency and item redundancy, resulting in a final set of 30 items which we called Work and Occupational Performance in ADHD (WOPA). The WOPA was further evaluated for floor and ceiling effects, internal consistency, and item-rest correlation. Additionally, associations of work performance were examined concerning work characteristics, ADHD symptom severity, indicators of functional impairments, executive function deficits, and internal emotional states. Discussion: The 30-item WOPA was developed based on empirically informed models, clinical considerations, and robust psychometric foundations. Subsequent clinical application is necessary to demonstrate its utility in clinical practice.