Synthetic hydroxyapatite (HAP) nanostructured materials have been considered as promising biomaterials due to their excellent biocompatibility. In this study, a magnetic nanocomposite consisting of HAP ultrathin nanosheets (UNs) and FeO magnetic nanoparticles (MNs) has been prepared using a rapid microwave-assisted route. The FeO magnetic nanoparticles are hybridized with HAP ultrathin nanosheets, which self-assemble to form a hierarchically nanostructured magnetic nanocomposite (HAPUN/MNs). The as-prepared HAPUN/MNs nanocomposite is characterized and investigated as a drug nanocarrier using hemoglobin (Hb) and docetaxel (Dtxl) as model drugs. The adsorption amount of Hb on the HAPUN/MNs nanocomposite increases with the increasing initial Hb concentration. The release of Hb from the HAPUN/MNs nanocomposite is essentially governed by a diffusion process. The HAPUN/MNs nanocomposite has a good sustained release profile for Dtxl, and shows good pH-responsive drug release properties, which can be explained by the gradual dissolution of HAP in a low pH value environment. The HAPUN/MNs nanocomposite has a high biocompatibility and also a high in vitro anticancer effect after loading Dtxl.