In this chapter attempts to produce a new spatial socio-economic composite index to gauge the incremental implications of the economic decay in the first instance accompanied by the environmental risks and threats on the socio-economic vulnerability/resilience in the mega-coastal metropolitan areas judging the capabilities of mega-coastal cities to absorb, adapt and then recover from exogenous economic shocks (economic decline processes and environmental risks). The model consists of three levels, starting with the impacts of losing the national and regional positioning of the main national maritime ports located in the coastal megacity accompanied by losing their role as the main city of tourism owing to the competition of other emerging tourism destinations and because of the climate-change risks. Second, we determine the implications of the above-mentioned risks on the cause of the decaying of the supporting and integrated economic activities. Third, we identify the extent to which the above-mentioned economic and functional decline has impeded and affected the socio-economic conditions. Finally, we produce and develop an aggregate and composite index measuring the vulnerability status quo based on ten different aspects of socio-economic and environmental decay spatially, to identify the most resilient and the most vulnerable districts. Therefore, the results of this chapter can be envisaged as an assessment technique guiding the formulation of adaptation policies seeking to combat the adverse consequences of socio-economic vulnerability in coastal megacities.