Stabilization of metal nanoparticles (MNPs) in polymeric matrices of different types has proven to be one of the most promising strategies to prevent their aggregation and to retain their properties. Polymer-stabilized MNPs (PSMNPs) and those based on polymer-metal nanocomposite materials are starting to find wide application in various fields of science and technology. In this paper, we demonstrate that metal-polymer nanocomposite membranes (MPNCMs) containing MNPs can easily be prepared in an ion-exchange such as, for example, sulfonated polyetherether ketone (SPEEK) matrix by using the polymeric membranes as nanoreactors for synthesis and to characterize the composition and structure of the formed MNPs. Metal ions (or metal ion complexes) are first incorporated into the polymeric matrix where they undergo reduction, leading to formation of corresponding MPNCMs. Since this technique allows successive metal loading-reduction cycles to be carried out, it enables synthesis of both monometallic and bimetallic (e.g., core-shell) MNPs. The proposed approach is illustrated by synthesis and characterization of MPNCMs containing both monometallic and bimetallic core-shell MNPs, formed by combinations of Pd, Pt, Co, Ni, and Cu, along with their application in electrochemical sensor and biosensor constructions.