In this work, we have performed a comprehensive investigation of the Ag⁺ concentration effect on the morphological, thermal, structural, and mid-infrared emission properties of novel oxyfluoride borosilicate glasses and glass ceramics containing both Ag nanoparticles and erbium-doped hexagonal NaYF₄ nanocrystals. The effect of Ag⁺ ions on the glass forming and crystallization processes was discussed in detail by glass structural analysis. It was found that the Ag⁺ concentration can affect the distribution of Na⁺ ion and bridge oxygen in boron-rich and silicon-rich phases, which induced the transformation between BO₃ triangles and BO₄ tetrahedra during crystallization process. In addition, there was a turning point when the doped Ag⁺ ion concentration reached its solubility in the borosilicate glass. Furthermore, the enhancement of the 2.7 μm emission and the reduction of the lifetime of the ⁴I_13/2 level were observed both in glasses and in glass ceramics, and its origin was revealed by qualitative and quantitative analyses of the Er³⁺-Ag nanoparticles (localized electric field enhancement) and Er³⁺-Er³⁺ (nonradiative resonance energy transfer) interactions within glasses and glass ceramics. Moreover, the high lifetime of the ⁴I_11/2 level (2.12 ms) and the peak emission cross section in 2.7 μm (6.8×10⁻²¹ cm²) suggested that the prepared glass ceramics have promising mid-infrared laser applications.