Although the circular economy (CE) concept is gaining traction and methods to assess companies’ CE-related aspects exist, there is no established CE assessment tool. In many cases, it is not clear how metrics or indicators included in extant CE assessment methods have been selected. To fill this gap, this paper presents a new instrument named Circularity and Maturity Firm-Level Assessment tool (CM-FLAT). The CM-FLAT has been developed starting from a transparent scientific basis, i.e., a recent systematic literature review and comprehensive collection of CE metrics. In addition, it targets the separate assessment of CE maturity, i.e., the presence of documented activities and practices laying the foundations for CE introduction, and circularity, i.e., attained CE-related performances. The development of the CM-FLAT has foreseen its formal evaluation by experts in the field of CE and sustainability, and its testing by a pilot group of companies from South Tyrol, Italy. The multiple verification activities have confirmed its usefulness and usability. Therefore, companies can now benefit from a tool capable of providing a comprehensive framework of factors and organizational areas affecting the introduction of the CE. This will be fostered by a computer-supported tool implementing the CM-FLAT, which represents the authors’ future work.