Rapid urbanization in sub-Saharan Africa is rapidly transforming land use and land cover (LULC) in a way that has enormous implications for both local climate and human well-being. This paper measures the spatiotemporal dynamics of LULC changes and its effect on land surface temperature (LST) and Urban Heat Island (UHI) strength in Ikeja, Lagos, Nigeria, the administrative capital of Africa’s largest megacity, over a ten-year period (2015–2025). Multi-temporal Landsat-8 and Landsat-9 imagery was classified using the Random Forest algorithm (overall accuracy: 91.4-93.2%) while LST was derived from the thermal band. Post classification change detection and transition-specific thermal analysis was integrated to ascertain the amount of warming attributable to individual LULC conversion. Results show a dramatic transformation of landscape: built-up area increased 44.7% (1,980–2,865 ha) while vegetation decreased 54.9% (1,256–567 ha). The area of water bodies decreased 41.6% (154 to 90 ha). The mean LST also increased 1.6°C (39.9°C to 41.5°C) with maximum values in 2025 exceeding 46.6°C, 8 times the global background warming rate. Vegetation-to-built-up conversions were the most significant contributors (+4.2°C) to warming, 2.6°C above background rates, followed by buildings, roads and impervious surfaces (+3.0°C), then the vegetation transition (+2.3°C). Attribution analysis shows that 85.3% of the total warming effect attributable to the loss of vegetation in urban areas. A critical cooling threshold was determined at 25% vegetation cover, and 12.7% of the study region now exceeds this threshold. Heat islands tended to remain strong in industrial, transport and commercial areas. This work offers the first-ever empirical estimate of transition-specific thermal impacts within a West African city, establishing that the loss of vegetation is the leading driver of UHI intensification. Vigilant and focused policy intervention, such as preservation of green space, targeted greening of hot spots and incorporation of heat stress criteria into the planning of cities, is needed to address dangerous warming and increase climate tolerance in the rapidly urbanizing tropical cities.