Global key drivers—e.g., climate change, COVID-19 outbreak—have initiated critical debates as to the sustainable future pathways of many regions around the globe. Among these fall islands, as distinct types of regions that are marked by insularity drawbacks and a mono-sectoral economic profile, grounded in mass tourism. Having as a case study area all Greek islands, this work addresses: sustainability concerns that are due to the mass tourism repercussions in these heritage-endowed territories; and efforts undertaken by the Greek insular municipalities to deploy, in a collaborative manner, strategic cultural tourism plans in support of the transition from a resource-intensive mass tourism pattern to natural and cultural heritage-led future developmental trails. Towards this end, a two-stream methodological approach is used that relies on: quantitative, spatially-defined data elaboration/interpretation and visualization, highlighting the dynamics of tourism development in island territories; and qualitative data on issues related to the aforementioned plans from all insular municipalities, gathered through a web-based questionnaire. Research results unveil the unsustainable spatial and developmental patterns of Greek insular contexts and the shortage of mainstream planning expertise for collaboratively establishing attractive cultural-tourism ensembles. These factors restrain sustainability achievements and the repositioning of Greek islands in the global scenery as authentic heritage-led destinations.