Microscopic studies of superconductors and their vortices play a pivotal role in understanding the mechanisms underlying super-conductivity(1-5). Local measurements of penetration depths(6) or magnetic stray fields(7) enable access to fundamental aspects such as nanoscale variations in superfluid densities(6) or the order parameter symmetry of superconductors(8). However, experimental tools that offer quantitative, nanoscale magnetometry and operate over large ranges of temperature and magnetic fields are still lacking. Here, we demonstrate the first operation of a cryogenic scanning quantum sensor in the form of a single nitrogen-vacancy electronic spin in diamond(9-11), which is capable of overcoming these existing limitations. To demonstrate the power of our approach, we perform quantitative, nanoscale magnetic imaging of Pearl vortices in the cuprate superconductor YBa2Cu3O7-delta- With a sensor-to-sample distance of similar to 10 nm, we observe striking deviations from the prevalent monopole approximation(12) in our vortex stray-field images, and find excellent quantitative agreement with Pearl's analytic model(13). Our experiments provide a non-invasive and unambiguous determination of the system's local penetration depth and are readily extended to higher temperatures and magnetic fields. These results demonstrate the potential of quantitative quantum sensors in benchmarking microscopic models of complex electronic systems and open the door for further exploration of strongly correlated electron physics using scanning nitrogen-vacancy magnetometry.