She worked with E. C. Pickering at Harvard College Observatory, measuring the brightness of stars on photographic plates. From her studies of Cepheid variables in the Small Magellanic Cloud, she established the period–luminosity relation (now also known as the Leavitt law) in 1912. In all, Leavitt discovered 2400 variable stars, more than half the total known in her lifetime. She also established the magnitudes of the stars in the original North Polar Sequence.