Cancer diagnosis and treatment have significantly advanced in recent years, resulting in improved survival rates. Conventional methods such as surgery, radiotherapy, and chemotherapy, along with newer approaches like hormonal therapy and immunotherapy, have achieved notable success. However, challenges like post-surgery relapses, potential adverse effects, limited benefits to specific patient groups, and low response rates remain major concerns. Despite technological advancements, cancer continues to pose a global challenge, with GLOBOCAN reporting 19.9 million cases and 9.7 million deaths worldwide in 2023. This chapter explores the innovative use of organic/magnetic core–shell nanoconstructs in cancer theranostics—a field combining therapy and diagnostic functions within a single platform. These sophisticated drug delivery systems merge therapeutic actions with imaging modalities like magnetic resonance imaging (MRI), near-infrared (NIR) imaging, magnetic particle imaging (MPI), and photoacoustic imaging (PAI). By incorporating polymers and magnetic nanoparticles (MNPs), these nanoconstructs offer targeted drug delivery, enhanced imaging, and hyperthermia treatment, thus improving treatment precision and efficacy. The physical properties of MNPs allow them to act as both imaging probes and drug delivery systems, offering real-time monitoring and personalized treatment options. This chapter delves into the design strategies, surface modifications, and functionalization of magnetic core–shell nanoconstructs, highlighting their biomedical applications, challenges, and future prospects in the realm of cancer theranostics.