A facile in situ synthesis approach and a size control strategy were established to obtain Ag₂S nanoparticles in polyimide (PI) composite coatings. Such Ag₂S nanoparticles in the composite coatings were characterized, and the effects of the as-obtained Ag₂S nanoparticles of different sizes on the mechanical and tribological properties of the nanocomposite coatings were investigated. Results indicate that the in situ synthesized Ag₂S nanoparticles exhibited good dispersibility and bimodal and multimodal size distribution in the nanocomposite coatings. The size of the Ag₂S nanoparticles can be effectively controlled by adjusting the substituent alkyl chain length of single-source precursor, and these Ag₂S nanoparticles exhibited superior improvement to mechanical and tribological properties of the nanocomposite coatings. More importantly, the Ag₂S nanoparticles with the proper grain size and bimodal size distribution provided the optimal mechanical and tribological properties for the nanocomposite coatings, and the excellent tribological properties were attributed to their outstanding mechanical properties and strong ability to form a homogenous and stable protective tribofilm.