The increasing use of mobile devices presents an unprecedented opportunity to collect activity data at a large scale. This data provides a near-continuous measure of activity across large cohorts over long periods of time, providing better temporal coverage and allowing for a more comprehensive investigation into the role of physical activity in health. At 23andMe, we are able to collect phone-based step count data through Apple’s HealthKit integration. With months of step count data collected on a cohort of over 10,000 individuals, we derive a number of activity phenotypes including daily step count, peak 60-minute cadence, and percent of daytime active. We then investigate both within- and between-individual variability of each phenotype, examining factors such as sex, height, device type, and day of the week. We also explore how device usage patterns impact inferences derived from phone-based step count data and use this information to investigate and quantify the bias associated with phone-based measures of activity. Our results can be used to inform strategies for analyzing mobile device-based longitudinal data and to guide the development of predictive models of health.