Multifunctional upconversion nanoparticles (UCNPs) that can be implemented in theranostic applications are particularly attractive scaffolds for precise drug delivery. However, most of the current methods for drug formulation are technically complicated, thereby impeding their use in the clinic. Here, we report on the preparation of a lipophilic cytotoxic prodrug-integrated and polyethylene glycol (PEG)-cloaked UCNPs scaffold through a facile one-pot supramolecular approach. By choosing 7-ethyl-10-hydroxycamptothecin (SN38)-derived prodrug 1 as a model chemotherapeutic, we show that this lipophilic prodrug can be feasibly self-assembled onto the surface of UCNPs, which are cooperatively solubilized by PEGylated phospholipids. The resulting SN38 prodrug 1-encapsulated UCNPs (designated 1@pUCNPs) produce a stable colloidal system in aqueous solution, making it suitable for intravenous injection. The SN38 drug loading capacity in pUCNPs is as high as ∼12.3 wt%, and a sustained drug release profile is observed, indicating that the drug payloads can be transported to targeted tumor sites via the enhanced permeability and retention (EPR) effect. Upconversion luminescence (UCL) imaging, including in vivo and ex vivo imaging, suggests that the drug-loaded pUCNPs remain stable in tumors over a long time and preferentially accumulate in tumors presumably via the EPR effect. Furthermore, the 1@pUCNPs show superior therapeutic outcomes compared with the clinically approved SN38 prodrug CPT-11 in the Bcap-37 mouse model of breast cancer. Collectively, our results demonstrate that pUCNPs facilely constructed in a one-pot self-assembly manner may be used as a versatile platform, enabling synchronous in vivo delivery of poorly water-soluble drugs and tumor imaging.