The Mn-doped CaCO<inf>3</inf>/PMMA nanocomposite were synthesized using an eco-friendly biomimetic method. This synthesis method is eco-friendly and hot-chemical-free. The current study suggests that instead of using large amounts of chemicals as a source, natural dolomite rock may be used to obtain pure nanocomposites without any impurities for extensive industrial requirements at a lower cost. The products were highly pure and crystalline in nature. The effects on the optical, thermal, and electrochemical properties of nano CaCO<inf>3</inf>/PMMA:Mn material were studied. The functional group and structural analysis were clearly explained and discussed using FTIR and XRD analyses. The products are in the crystallite size range of 20–40 nm and have a rhombohedral structure. The addition of PMMA to the CaCO<inf>3</inf> matrix resulted in the overlapping of multiple color centers, indicating a decrease in the energy gap. Higher thermal stability (837 °C) was achieved for CaCO<inf>3</inf>/PMMA: Mn (0.12 M) nanocomposite than in other literature values. Mn (0.12 M)-doped CaCO<inf>3</inf>/PMMA has a high specific capacitance, good rate capability, and outstanding cyclic stability. The specific capacitance of CaCO<inf>3</inf>/PMMA: Mn (0.12 M) nanocomposite was higher than other literature values. These techniques suggest that Mn doping and PMMA matrix decoration can enhance the electrochemical performance of CaCO<inf>3</inf> electrolytes. The cyclic stability of CaCO<inf>3</inf>/PMMA: Mn (0.12 M) nanocomposite after 2000 cycles at 5 Ag− 1 showed better cycle endurance, with a high retention of 93.2%. © 2023 Elsevier B.V., All rights reserved.