Although previous attempts for enhancing the buckling resistance of cold-formed steel (CFS) built-up beams through intermittent stiffening, external stiffening and adoption of concrete as composite sections were successful, they may compromise the lightweight constructional feature of these elements. This paper presents a test program aimed at developing CFS composite built-up beams with superior buckling performance and low self-weight for ensuring lightweight construction. Unlike concrete, lightweight stiff boards of timber and glass fibre reinforced polymer (GFRP) were sandwiched between the CFS channels in the web and flange region to construct the lightweight composite beams. All of the beam specimens were subjected to four-point bending with simply supported boundary conditions. Peak strength, load-deformation curves, initial stiffness, strength-to-weight ratios, failure modes and the normalized yield and plastic moment strengths were compared. This study established the structural feasibility of developing lightweight CFS composite built-up beams from timber and GFRP boards, indicating that the flexural strength and stiffness are significantly improved by 150% and 80%, respectively, as compared to the unstiffened CFS built-up beams. Moreover, the studied composite beams were shown to attain their full yield moment capacities, and in some cases more than 90% of their plastic moment capacity, resulting in built-up members with a higher structural efficiency.