Quantification and analysis of urban greenness is important in evaluating the role of urban trees in carbon sequestration and regulating land surface temperature of an urban environment. Landsat 8 satellite data was used to evaluate the effects of urban heat island based on land surface temperature (LST) in the affluent suburb of Rosebank in the north and the previously marginalized township of Soweto south of Johannesburg. It was found that the LST of Soweto was 2.58 °C hotter than Rosebank, suggesting that high-density areas with less street trees and parks are hotter than low-density suburbs. The Pearson's correlation coefficient computed between the LST and the Normalized Difference Built-up Index (NDBI) or the Normalized Difference Vegetation Index (NDVI) for the two areas, Rosebank and Soweto showed a positive correlation of 0.92 and 0.98) and a negative correlation of −0.91 and −0.99 respectively. The correlation between the NDVI and NDBI also showed negative values of −0.90 and −0.85, respectively. Based on the global bench mark for safe solar UVI, the residents of Soweto are likely to be at risk of skin and eye damage due to prolonged exposure to ultraviolet radiation than the residents of Rosebank. Thus, this study has effectively computed and evaluated the land surface temperatures of the two study areas from Landsat 8 satellite data, an information which can help potential vegetation cover programs to improve existing disparity in urban greenness in the city of Johannesburg. •11.15% of the Soweto is affected by urban heat island compared to 1.77% in Rosebank.•The land surface temperature in Soweto is 2.48 °C hotter than in Rosebank.•Vegetation cover reduces effects of urban heat island while built-up areas increases it.•Residents of areas with high to very high UV exposure are likely to suffer from health problems.