In this work, the effect of montmorillonite (MMT) nanoclay on the structure, morphology, thermal, mechanical, and barrier properties of the resulting low density polyethylene (LDPE) nanocomposites prepared via melt extrusion was investigated. The level of exfoliated dispersion of the MMT nanoclay in the prepared LDPE-MMT nanocomposite was confirmed using transmission electron microscopy (TEM). The relationship between the resulting morphology and the thermal, mechanical, and barrier properties as a function of the MMT content was evaluated. The results showed that incorporating >3 wt.% of MMT nanoclay produced significant changes in the morphology of the LDPE-MMT nanocomposite in that the segregated matrix adopted an oriented arrangement of exfoliated MMT platelets. Thermogravimetric analysis (TGA) showed that the thermal stability of LDPE improved significantly as a result of MMT nanoclay incorporation. Furthermore, differential scanning calorimetry (DSC) analysis indicated that increasing clay content above 3 wt.% effectively reduces the crystallinity of LDPE/MMT composites through the suppression effect. The tensile strength of LDPE increased gradually with an increased content of MMT nanoclay and the maximum value of 16.91 N/mm² was obtained at 10 wt.% MMT content. This value represents a 40.89% increase relative to the tensile strength of the pristine LDPE. Barrier properties of LDPE and LDPE-MMT nanoclay composites were assessed by examining the permeability with respect to oxygen and water vapor. As the content of MMT nanoclay was increased to 10 wt.%, the permeability of the nanocomposite films to oxygen and water vapor notably decreased to 42.84% and 26.25%, respectively.