Zn1-xMgxO nanocomposites were synthesized by a modified sol-gel method. The effect of increasing Mg content on structural, optical properties, and antibacterial activity was studied. The structure and physical properties of the Zn1-xMgxO nanocomposites were characterized by thermogravimetric (TG) analysis, Fourier-transform infrared spectroscopy (FT-IR), X-Ray Diffraction (XRD), transmission electron microscope (TEM). The optical properties were investigated by UV-visible spectroscopy and Photo luminesce spectra (PL). The thermal properties and the weight stability were studied for the samples using TG-Analysis to determine the best annealing temperature at 550 °C. The XRD result confirms the Clear indications for segregation into a hexagonal and a cubic phase are found for samples having magnesium content between 0.2 ≤ x ≤ 0.8, according to Scherer's formula, the average particle size of the synthesized nanocomposites is in the range of 16.32- 25.56 nm. TEM images showed hexagonal, irregular, and spherical shapes. The calculated band gap energies (Eg) of the (Zn1-xMgxO) nanoparticles were changed from 3.188 eV to 3.31 eV by increasing of Mg concentration in the Zn1-xMgxO nanocomposites. The XRD and UV-vis analysis results indicated that the physical characteristics of Zn1-xMgxO nanocomposite were dependent on increasing the incorporation of Mg2+ ion concentration. The antibacterial assessment illustrated that The ZnO-MgO nanocomposite showed excellent antibacterial activity, the inhibition zoon increases from 12 mm for pure ZnO to 15 mm for (Zn1-xMgxO) nanocomposite with Mg ratio (x=0.4). The result concluded that Zn1-xMgxO nanocomposites have significant potential for antibacterial applications.