CaCO<inf>3</inf>/PMMA nanocomposites doped with magnesium were synthesised through a green approach (biomimetic method), using a CaMg(CO<inf>3</inf>)<inf>2</inf> (natural dolomite rock). The present products were characterised using various spectroscopic techniques. UV–Vis (270–276 nm) and PL (426–434 nm) results indicate that the products exhibit good optical properties. The optical band gap energy values range from 1.9 to 2.9 eV. XRD confirmed the presence of the calcite phase in the prepared samples, indicating a rhombohedral crystalline structure of CaCO<inf>3</inf> in the calcite polymorph, with crystallite sizes of 42, 35, and 37 nm for the different product concentrations. FTIR analysis confirmed the presence of metal oxide (Mg-O) with peaks around 423 cm-1. FESEM and HRTEM results show the product has a spherical morphology with an average particle size of 40 nm. XPS analysis confirmed the presence of Ca, C, O, and Mg. The degradation results show that 0.02 mol Mg-doped CaCO<inf>3</inf>/PMMA nanocomposites exhibit 86.23 % (k = 0.0103 min-1) degradation of malachite green (MG) dye within 150 min. Antibacterial activity (in vitro), 0.06 mol Mg-doped CaCO<inf>3</inf>/PMMA nanocomposites showed higher efficacy against both gram-positive and gram-negative bacteria (S. aureus, S. pyogenes, K. pneumoniae, and Escherichia coli). The achieved maximum zone of inhibition was 21.25 mm at a dose concentration of 100 μg/ml. These findings demonstrate that Mg-doped CaCO<inf>3</inf>/PMMA nanocomposites are ideal as sustained photocatalysts and suitable for biological applications. © 2025