This study investigates the strength parameter of a novel composite material termed Lightweight Cemented Soil (LCS), which is a mixture of Black Cotton Soil (BCS) reinforced with cement and expanded polystyrene (EPS) beads of varying densities. The BCS is stabilized with a constant cement content, maintaining a cement-to-BCS (C/BCS) ratio of 5%. EPS beads are introduced into the mix at two different densities and two distinct mix ratios (EPS/BCS) by mass. Initially, these mixtures’ optimum moisture content and maximum dry density were determined using standard proctor tests. Subsequently, the Unconfined Compression Test (UCS) and California Bearing Ratio Test (CBR) were performed at the identified maximum dry density values. Results reveal that the EPS density and the EPS/BCS mix ratio significantly influence the composite material’s density, UCS, and CBR values. The simultaneous variation in EPS density and EPS content in this study highlights its novelty. The significant reduction in overall density achieved by incorporating EPS beads into the cement-stabilized BCS suggests that this composite material is ideal for constructing lightweight embankments, particularly in areas with poor load-bearing subgrades. This research contributes to developing lightweight construction materials with improved load-bearing capacities.