A general method has been developed for the fabrication of highly dispersed and thermally stable bimetallic Au–Ag nanoparticles (NPs) stabilized on the inner wall of a mesoporous silica shell. In our approach, gold particles were formed in the first step on carbon spheres decorated with Sn2+ cations. Upon Ag+ adsorption and reduction by L-ascorbic acid in the second step, specific nanoparticles with a gold–silver alloy core and a silver nanoshell have been formed. Important evidence of the core–shell configurations of the bimetallic Au–Ag nanoparticles were clearly characterized by UV-vis, TEM and HAADF-STEM observations combined with elemental mapping and line scans. The mesoporous silica outer shell was obtained through the hydrolysis and condensation of the precursors tetraethoxysilane (TEOS) in a basic condition and cetyltrimethylammonium bromide (CTAB) as a structure-directing agent. On this basis, the nanoreactors were fabricated after calcination, which further serves as a nanoreactor for the reduction of p-nitrophenol. Furthermore, such particles have been found to be thermally stable and their sizes remain substantially unchanged even upon calcination in air at 500 °C and a reduction treatment in H2. Potentially, the method can be developed into a general approach to synthesize other highly dispersed and thermally stable bimetallic nanoparticles stabilized on the inner wall of a mesoporous silica shell.