A novel reference electrode was fabricated via modification of a Pt electrode with a solid-state mixed matrix membrane generated by mixing casein phosphopeptides as Ca2+ and Cl− ionophores, multi-walled carbon nanotubes (MWCNTs) as an electrical support (inorganic phase) and micro hybrid resin as binder (organic phase) with the weight ratios of 25:25:50 (wt.%), respectively. The selectivity of casein toward Ca2+ during formation of casein phosphopeptides–Ca2+ nanocomplex (average diameter: ∼250 nm) was evidenced using UV–Vis spectrometry. Linear adsorption behavior was also estimated for the fabricated nanocomposite. Potentiometric technique revealed the strong adsorptivity of the nanocomposite to the chloride ion. The fabricated reference electrode showed adequate stability in various ionic strengths generated in the presence of ions such as Na+, K+, Mg2+, Cl−, I−, I3−, SO42−, PO43−, ClO4−, etc. The non-polarization behavior of the fabricated reference electrode was confirmed during passing at least 10.0 mA electrical current thorough the electrode. The fabricated half-cell of the reference electrode with notation of “Pt | KCl(satd.) | Ca2+ (1.0 μm) nanocomposite membrane|” had a fixed reduction potential equal to −0.070 ± 0.002 V vs. standard hydrogen electrode. The electrical stability of the electrode was originated from irreversible adsorption behavior of the nanocomposite to the Cl− as well as from the stability of casein phosphopeptides–Ca2+ nanocomplex during the conditioning steps. The reliability of the fabricated reference electrode was studied during testing in a three-electrode system including two Pt disks as working and counter electrodes during evaluating the nerstian behavior of cyclic voltammetry of 1.0 mM Fe(CN)64−/3− solution with anodic and cathodic peak potential difference (ΔV) of 59.0 ± 0.1 mV. The lifetime of the fabricated reference electrode was estimated to at least 12 months.