This paper estimates the link between population density and COVID-19 spread and severity in the contiguous United States. To overcome confounding factors, we use two Instrumental Variable (IV) strategies that exploit geological features and historical populations to induce exogenous variation in population density without affecting COVID-19 related deaths directly. We find that density has affected the timing of the outbreak in each county, with denser locations more likely to have an early outbreak. However, we find no evidence that population density is linked with COVID-19 cases and deaths. Using data from Google, Facebook and the US Census, we also investigate several possible mechanisms for our findings. We show that population density can affect the timing of outbreaks through higher connectedness of denser location. Furthermore, we find that population density is positively associated with proxies of social distancing and negatively associated with the age of the population, highlighting the importance of these mediating factors.