Despite being the places where most people prefer to live and work, cities face significant challenges. Triggered by the COVID-19 pandemic, the 15-min city concept has emerged as the new urban planning paradigm that aims to cope with intense urban challenges by bringing the idea of living locally to the forefront. Although the concept of the 15-min city has a great potential to enhance the quality of life, its implementation could never be realised unless its theoretical principles are translated into measurable and evaluable dimensions. Thus, the current paper introduces a methodological framework that quantitatively assesses cities' compliance with the 15-min city concept through a sophisticated, holistic fuzzy index, the 15-MCiX. The proposed methodological framework was applied in the city of Thessaloniki, Greece, and highlighted that Thessaloniki largely embraces the concept of the 15-min city, achieving an average 15-MCiX score of 0.85 out of 1.0. Thus, a more ambitious yet meaningful target, such as the 10 or 5-min threshold, should be considered in dense urban environments like those in Europe. Finally, the pilot implementation underlined the great reliability, usability, and transparency of the proposed methodological framework, as well as its high transferability that could make it a valuable tool worldwide.