Atomic force microscopy was employed to probe the mechanical properties of surface-charged polymethylmethacrylate (PMMA)-based terpolymer and composite terpolymer core-silica shell particles in air and water media. The composite particles were achieved with two different approaches: using a silane coupling agent (composite A) or attractive electrostatic interactions (composite B) between the core and the shell. Young's moduli (E) of 4.3 +/- 0.7, 11.1 +/- 1.7, and 8.4 +/- 1.7 GPa were measured in air for the PMMA-based terpolymer, composite A, and composite B, respectively. In water, E decreases to 1.6 +/- 0.2 GPa for the terpolymer; it shows a slight decrease to 8.0 +/- 1.2 GPa for composite A, while it decreases to 2.9 +/- 0.6 GPa for composite B. This trend is explained by considering a 50% swelling of the polymer in water confirmed by dynamic light scattering. Close agreement is found between the absolute values of elastic moduli determined by nanoindentation and known values for the corresponding bulk materials. The thickness of the silica coating affects the mechanical properties of composite A. In the case of composite B, because the silica shell consists of separate particles free to move in the longitudinal direction that do not individually deform when the entire composite deforms, the elastic properties of the composites are determined exclusively by the properties of the polymer core. These results provide a basis for tailoring the mechanical properties of polymer and composite particles in air and in solution, essential in the design of next-generation abrasive schemes for several technological applications.