In the present study, eco-friendly polylactic acid (PLA)-high-density-polyethylene (HDPE)-based polymer reinforced with magnesium oxide was fabricated. Experimental findings reveal that the resultant composite exhibited enhanced tensile and flexural properties as well as higher glass transition temperature and degradation temperature compared to neat PLA-HDPE polymer. For instance, tensile strength and modulus of composite reinforced with 5 wt.% MgO improved by 22.27% and 11.98%, respectively, while the flexural modulus of the composite was enhanced by 16.2% in comparison to PLA-HDPE composite. The improved flexural strength with the addition of MgO indicates the ability of MgO to improve the ability to cope with transverse load (and reduce deflection) in the composite. Findings from the present study were also compared with data reported in literature to provide an insight on the mechanical behaviour of PLA-HDPE-MgO composites.