The goal of this project was to develop an evidence-based method to assess the ability of disabled persons to manage federal disability payments. This paper describes the development of the FISCAL (Financial Incapability Structured Clinical Assessment done Longitudinally) measure of financial capability. The FISCAL was developed by an iterative process of literature review, pilot testing, and expert consultation. Independent assessors used the FISCAL to rate the financial capability of 118 participants (57% female, 57% Caucasian) who: received Social Security disability payments, had recently been treated in acute care facilities for psychiatric disorders, and who did not have representative payees or conservators. Altogether, 48% of participants were determined financially incapable by the FISCAL, of whom 60% were incapable due to unmet basic needs, 91% were incapable due to spending that harmed them (e.g. on illicit drugs or alcohol), 56% were incapable due to both unmet needs and harmful spending, and 5% were incapable due to contextual factors. As expected, incapable individuals scored higher on a measure of money mismanagement (p < .001) compared to capable individuals. Inter-rater reliability for FISCAL capability determinations was very good (Kappa = .77) and inter-rater agreement was 89%. In this population the FISCAL had construct validity; ratings demonstrated good reliability and correlated with a related measure. Potentially, the FISCAL can be used to validate other measures of capability and to help understand how people on limited incomes manage their funds.