With ~57% of the world population living in cities, the global urban population is increasing at an alarming rate, which further stimulates the urbanization process. Consequently, the increasing impervious surfaces in cities and associated variabilities in local/regional climatic characteristics pose several challenges to citizens (i.e., heat-related health issues, higher energy demands, and flooding among many others). Currently, cities contribute 75% of Green House Gases emissions, which is further worsening climate change impacts through global warming. Pakistan, the 6th most populated country globally, with ~220 million people, is among the top 10 most-affected nations vulnerable to climate change. Hence, studies addressing climate variability in local geographical regions have important implications to address the adverse urbanization-associated challenges, such as sustainability of the land resource and mitigating urban heat island (UHI) impacts in the context of climate change mitigation/adaptation. Due to temperature differences between urban, suburban, and rural areas, mapping city zones prone to the UHI effect is essential to provide actionable references. In connection with this, the present study analyses 15 megacities in Pakistan regarding their temperature variability in response to built-up area increment and highlights heat stress zones using the Urban Thermal Field Variance Index (UTFVI). The cloud-computing-based Google Earth Engine platform is employed to explore spatial–temporal variation in Land Surface Temperature (LST), which further leads to the identification of top-15 cities in terms of LST increase and the further evaluation of UTFVI for each city. The findings of this study suggest that the strongest UTFVI zones are concentrated around city-core areas, which are pure impervious surfaces with little or no green space. Moreover, in the last three decades (1990–2020), most of the weak and strong-strength UTFVI areas have been converted into the strongest strength primarily because of a rapid increase in the built-up areas. The findings of this study can help urban policymakers to identify priority intervention areas and design/implement strategies to counter the UTFVI and associated challenges. With proper land-use planning and on-time policy implementation, people residing in higher UTFVI zone areas can be safeguarded from noxious heatstroke-like health consequences along with mitigating and adapting to changing environmental conditions in cities.