Free-standing 2D nanoassemblies are a new class of advanced functional materials that can integrate the unique properties of their individual components. Herein, we report a facile template-free solvothermal strategy to manipulate the assembly of structurally distinct building blocks into free-standing 2D quasi-nanosheets. The obtained 2D nanoassemblies are magnetic in nature with thickness in the range of tens of nanometers and lateral dimensions of several micrometers. They showed exceptionally high stability and can even remain intact in any solvent for months without any significant disassembly. The magnetic quasi-nanosheets were further used to remove polystyrene (PS), a model microplastic pollutant, from water. The robust phosphomolybdic acid (PMA) cluster-based hybrid exhibited outstanding PS removal performance because of the unique quasi-nanosheet architecture and multiple coordination sites of the redox-active PMA clusters. Overall, our developed co-assembly and subsequent water purification strategy effectively tackles various limitations associated with traditional assembly and water purification strategies.