Air pollution and heat pose considerable threats to public health, particularly in urban areas and under the effects of climate change. The well-designed urban form has mediating effects on the atmospheric environment (such as heat and air pollution) and promotes human health. The extant literature has overlooked these mediating effects of urban form on pneumonia mortality. The present study used urban-scale data from 19 counties in Taiwan and partial least squares modeling to identify the crucial effects and pathways of urban form on pneumonia mortality via air pollution and heat. In particular, the model considered the effects of the characteristics of urban form (i.e., urban size, land use mix, and urban sprawl), social situation (i.e., smoking behavior, elderly ratio, and medical resources), and atmospheric environment (i.e., heat and air pollution), as well as urban industry (i.e., industrial level) and disease (i.e., HIV morbidity), on pneumonia mortality. Minimization of urban sprawl and urban size and optimization of land use mix were found to lower pneumonia mortality, and minimization of urban size was the most crucial characteristic of urban form correlated with pneumonia mortality. Industrial level, smoking behavior, elderly ratio, medical resources, HIV morbidity, heat, and air pollution also had influences on pneumonia mortality. This is the first study to consider the effects and pathways of urban form characteristics on pneumonia mortality. The findings demonstrate that appropriate planning, design, and policy may reduce pneumonia mortality.