High-spatial-resolution magnetic imaging has driven important developments in fields ranging from materials science to biology. However, to uncover finer details approaching the nanoscale with greater sensitivity requires the development of a radically new sensor technology. The nitrogen-vacancy (NV) defect in diamond has emerged as a promising candidate for such a sensor on the basis of its atomic size and quantum-limited sensing capabilities. It has remained an outstanding challenge to implement the NV centre as a nanoscale scanning magnetic probe at cryogenic temperatures, however, where many solid-state systems exhibit non-trivial magnetic order. Here, we present NV magnetic imaging down to 6 K with 3 μT Hz(-1/2) field sensitivity, and use the technique to image vortices in the iron pnictide superconductor BaFe2(As0.7P0.3)2 with critical temperature Tc = 30 K. The expansion of NV-based magnetic imaging to cryogenic temperatures will enable future studies of previously inaccessible nanoscale magnetism in condensed-matter systems.