Despite the continuous developments in the synthesis of noble metal nanoparticles, the uniformity of particle size distribution still represents a critical aspect. A fast and homogeneous nucleation is a key requirement to achieve a monodisperse particle size distribution and in this scenario, the application of alternative energy sources may constitute a winning strategy for the development of highly active nanocatalysts with unique properties. Here we present several approaches to control the synthesis of Ag nanoparticles stabilized by an anionic template, and the results evidence the advantages of adopting unconventional heating techniques such as microwave heating. The fast and selective electromagnetic heating strongly reduced the nucleation and growth times, impacting on the homogeneity of the resulting particle size distribution. In this work, we have carried out the microwave-assisted synthesis of Ag nanoparticles and the resulting nanoparticles were compared to those synthesized under conventional heating using an oil bath, showing that the differences in temperature profile and heating rates between the two synthesis pathways had a clear effect on the size distribution of the resulting nanoparticles as well as on their stability under long term storage. Finally, the synthesized ultra-small Ag nanoparticles were deposited on a mesoporous substrate, reducing undesired Ostwald ripening and facilitating their reusability. This nanocatalyst was adopted for the abatement of 4-nitrophenol, a well-known carcinogenic pollutant with adverse effects on human beings and aquatic life. The catalytic results confirm the high activity of the catalyst thanks to the high dispersion achieved afforded by ultra-small Ag nanoparticles and the accessibility provided by the wide SBA-15 mesoporous channels.