Photodynamic therapy (PDT) is a novel technique that has been extensively employed in cancer treatment; it utilizes reactive oxygen species to kill malignant cells. However, poor performance of the photosensitizer itself, limited penetration depth and the overexpression of glutathione (GSH) in cancer cells are the major obstacles facing the actual clinical application of PDT. Inspired by the challenges mentioned above, here we propose multifunctional nanoparticles utilizing mesoporous manganese silicate (MnSiO)-coated upconversion nanoparticles (UCNPs) as nanocarriers for loading highly fluorescent graphitic-phase carbon nitride quantum dots (g-CN QDs) to simultaneously act as a photosensitive drug and imaging agent. Surface modification of the nanoparticles with polyethylene glycol (PEG) endows the samples (denoted as UMCNs-PEG) with excellent biocompatibility and long-term in vivo circulation. Taking advantage of the inherent performance of the as-synthesized nanoparticles, multimodality imaging, including upconversion luminescence (UCL), computed tomography (CT) and magnetic resonance imaging (MRI), has been achieved; this is conducive to providing effective treatment information by real-time monitoring. In vivo photodynamic therapy to achieve effective tumor inhibition was then realized without inducing significant toxicity to treated mice. As a result, this work provides a novel paradigm with highly integrated functionalities which not only exhibits excellent prospects for imaging-guided photodynamic anticancer therapy but also encourages further exploration of new types of multifunctional nanoparticles for biomedical applications.