Information on individuals’ mobility—where they go as measured by their smartphones—has been used widely in devising and evaluating ways to respond to COVID-19, including how to target public health resources. Yet little attention has been paid to how reliable these data are and what sorts of demographic bias they possess. A new study tested the reliability and bias of widely used mobility data, finding that older and non-White voters are less likely to be captured by these data. Allocating public health resources based on such information could cause disproportionate harms to high-risk elderly and minority groups.