In this paper, we report on an effective and easy method to in situ synthesize luminescent Ag NCs in carboxylic group-rich electrospun polymer fibers to form multifunctional composite polymer fibers. In present study, we electrospun a kind of poly(methyl methacrylate)–poly(methacrylic acid) (PMMA–PMAA) nanospheres to prepare carboxylic group-rich electrospun polymer fibers. Through an ion exchange process between the hydrogen ions of carboxylic groups and Ag ions, Ag ions were captured and enriched in the fibers and then they were in situ reduced to Ag(0) by a photochemical reduction reaction. Consequently, novel Ag NC composite polymer fibers were achieved and they exhibited a bright pink luminescence. This in situ growth method endows Ag NCs with an excellent, long-term optical stability due to the protection of the solid polymer matrix. In addition, we found that these luminescent fibers presented antibacterial activity, which can be a promising candidate as a class of antibacterial membrane materials.