This study examines the structural, thermal, and morphological characteristics of polyethylene glycol (PEG) reinforced with functionalized multi-walled carbon nanotubes (F-MWCNTs) at various concentrations (0.5–1.5 wt.%). FTIR analysis verified successful incorporation and interfacial interactions between PEG and functionalised MWCNTs, as evidenced by shifts in –OH and C–O–C vibrational bands, indicating hydrogen bonding. Thermogravimetric analysis (TGA) showed enhanced thermal stability of the nanocomposites, with Tonset, Tmax, and Tf values steadily increasing with MWCNT content, alongside higher residual mass, confirming the thermal shielding effect of the nanotubes. DSC results revealed a notable rise in glass transition temperature (Tg) and a reduction in melting peak intensity, indicating restricted chain mobility and decreased crystallinity due to MWCNT/PEG interactions. SEM micrographs displayed a transition from a smooth surface in pure PEG to a rough, textured morphology in the nanocomposites, suggesting better dispersion and interfacial adhesion of MWCNTs within the matrix. XRD patterns corroborated these findings, showing increased crystallinity and distinct MWCNT-related peaks in the composites, with heightened peak intensity at higher filler loadings. Overall, the synergistic effect of MWCNTs significantly improves the thermal, structural, and morphological properties of PEG, making these nanocomposites promising candidates for advanced material applications.