Mesoporous magnetic colloidal nanocrystal clusters (MCNCs) are featured with high magnetization, adequate surface area, excellent colloidal stability, good biocompatibility, and acid degradability. It is thus highly anticipated that MCNCs can serve as vehicles for target drug delivery. Herein, the mesoporous MCNCs stabilized by poly(gamma-glutamic acid) (PGA) were fabricated by the modified solvothermal route, showing a high specific surface area (126.4 m2/g), strong magnetic response (63 emu/g) and appropriate mesoporosity including a large pore volume (0.27 cm3/g) and accessible pore size (8.1 nm). Docetaxel (DOC) was then loaded in the resultant MCNCs using the nanoprecipitation method, and a high drug loading capacity was achieved up to 24 wt%. The chemotherapeutic effect and mechanism of DOC-MCNC conjugates in bladder cancer was evaluated in vitro. A series of analyses for cell uptake, cell viability, cell cycle, cell apoptosis and some cell proteins were performed by transmission electron microscopy, MTT assay, flow cytometry, cell nuclei staining, Annexin V staining assay, western blot assay and caspase-3 activity assay, respectively. The results demonstrated that DOC-MCNC conjugates enhanced the inhibitory effect by hampering mitoschisis and increased the apoptotic effect by changing the expression of apoptosis-related proteins in T24 cells, substantially proving their remarkable efficiency in treatment of bladder cancer.