Sustainable urban mobility has been the epicenter both at the scientific and administrative level during the last decades, with a high number of relevant research projects, awareness campaigns, and other initiatives taking place at the local, national, and international level. However, many urban areas have so far achieved limited results in this direction because of political, institutional, organizational, technological, infrastructural, and socio-economic barriers as well as unforeseeable (e.g., COVID-19) conditions. The overall aim of the present research study is to support policy-making by proposing a methodology that identifies and prioritizes the sustainable mobility barriers for a specific urban area, with a view to developing effective policies. Towards this purpose, this work provides, in the first phase, a comprehensive inventory of barriers based on a literature review. In the second phase, a methodology using as a basic scientific tool a modified Delphi-AHP is proposed for the adaptation of this inventory to a specific urban area and for both the evaluation and prioritization of sustainable mobility barriers. The whole process is then applied in Thessaloniki, Greece, a European city suffering from many problems related to sustainable mobility. The above pilot application confirms that this approach can be integrated as a supporting tool in the first steps of sustainable urban mobility plans (SUMPs).