This study presents the development of the Working Alliance of Mandated Clients Inventory (WAMCI). The goal of the WAMCI is to measure positive and negative factors of the working alliance from two perspectives: the probationer and the probation officer (PO) in the Dutch context. Data from 302 probationers and 267 POs of all three probation services in the Netherlands were used. Based on two existing and validated instruments and with the addition of supplemental items specific for the Dutch probation context, this new inventory was developed to capture the Dutch and European practice. An initial psychometric evaluation was conducted with this new inventory. A principal components analysis and a structural equation analysis led to a four-factor solution that provided the best fit for the PO and the probationer versions of the WAMCI. The internal consistency of the established scales Trust, Bond and Goal-Restrictions was sufficient, but the internal consistency of the established scale Reactance was not satisfactory. The construct validity of the WAMCI was supported by correlations with other constructs. Higher scores on the WAMCI were related to higher scores on general relationship satisfaction. The Goals-Restrictions subscale was moderately associated with a measure of internal motivation.