One significant effect of urbanization is the modification of natural surfaces, local environment, and thermal comfort. Farmland and agricultural land in Klang Valley conurbation, Malaysia have been replaced with engineered surfaces and infrastructures to accommodate the rapid population growth witnessed in the past decades. To understand the current environmental conditions of the conurbation, numerical Weather Research and Forecasting (WRF) model coupled to the Urban Canopy Model (UCM) was used. A model evaluation conducted against a network of observations showed an overall good performance of the model in predicting near-surface meteorological parameters. Also, an examination of spatiotemporal variability of urban parameters revealed a diurnal dependence of human physiological thermal conditions on urbanization level. A maximum urban heat island intensity (UHII) of ~2.64 °C was observed. In an effort to investigate the effect of urbanization level on the extremities of urban climatic conditions, two different experiments with varying urban/vegetation fractions were further simulated. The latter results indicate that urbanization level has a significant effect on the extremities of urban climatological parameters and spatial extent of the induced impacts. Furthermore, the effect of urbanization level on the mean urban outdoor thermal discomfort (UOTD) level was significant (with an increase of 0.7 °C, 0.64 °C and 0.04 observed) for temperature humidity index (THI), effective temperature index (ETI) and relative strain index (RSI), respectively. However, a non-significant (at 95% level) effect of urbanization level on the extremities of UOTD indices was observed.