We developed a psychometric questionnaire to quantify the mental performance fitness of individuals who perform critical actions under pressure. The development utilized a combination of a traditional literature review and AI synthesis. A traditional literature review was employed to evaluate published mental toughness assessments and identify standard components. The AI synthesis utilized a targeted search of functional components within the three most commonly reported performance-supporting mental modalities in professional sports: performance routines, meditation, and cognitive behavioral therapy. The traditional literature review revealed a high level of variation in definitions of constructs and the selection of components in mental toughness questionnaires. The AI synthesis, on the other hand, identified a compact set of components and provided a clear rationale for the construct being measured. The AI synthesis demonstrated that skills must be assessed separately from cognitive-emotional attributes, leading to the development of a three-domain mental fitness index. The assessment literature was then used to devise frequency-based weighting across the domains and create a five-point Likert scale index. The resulting mental performance fitness index is currently used in conjunction with outcome-based performance assessments to validate its use in tracking the impact of mental performance training among high-performance athletes.