As public health risks resulting from urban heat in cities increase due to urbanisation and climate change, there is a pressing need to design strategies for urban heat mitigation and ensure that future development is climate sensitive. Heat stress in cities is mainly influenced by four factors: the built form, natural and vegetated form, human urban activities, and regional geographic settings (e.g. topography and distance to water bodies). The first two factors can be modified and redesigned as urban heat mitigation strategies (e.g. changing the albedo of surfaces, replacing hard surfaces with pervious vegetated surfaces, or increasing canopy cover), whereas while human activities can be modified, the impacts of these can be difficult to quantify, and regional geographical settings of cities cannot be modified. However, when evaluating the effectiveness of urban heat mitigation strategies based on modifications to the built and natural forms, it can be difficult to separate their impacts from the interactions of the geographic influences. To address this, we performed a comprehensive urban form analysis, covering the full range of realistic built and natural forms (building density and height, roads, grass, and tree density and height) in cities, along with a combination of mitigation strategies, to determine the importance and influence of each on thermal performance. We show that during the daytime, higher air temperatures and Universal Thermal Climate Index temperatures are strongly driven by increased street fractions, with air temperatures increasing up to 10 and 15C as street fractions increase to 80 and 90%. Reductions in air temperature of 5C are seen with increasing grass and tree fractions from none to complete coverage. Similar patterns are seen with the Universal Thermal Climate Index, with increasing street fractions of 80% and 90% driving increases of 6 and 12C. We then scale up the results to produce city-wide heat maps of several Australian cities showing the impact of present day urban form. The resulting method allows mitigation strategies to be tested on modifiable urban form factors isolated from geography, topography, and local weather conditions, factors that cannot easily be modified.