<p>This study presents the development of sustainable polyester resin hybrid composites by incorporating industrial waste fly ash cenosphere and dolomite as fillers. Fly ash cenosphere, treated with (3-Aminopropyl) triethoxy silane, and untreated dolomite, with mean particle sizes of 63 and 14&#xa0;µm, respectively, were used. The hybrid composites containing functionalized fly ash cenosphere (FFAC)/dolomite (DO) were prepared using a filler concentration of 10 wt% in the ratios of 75:25, 50:50, and 25:75 and composites were tested for thermal, tensile, flexural, and viscoelastic properties. FTIR spectroscopy confirmed successful surface modification of cenosphere particles. The thermogravimetric analysis confirmed the improvement in thermal stability with the addition of fillers. 2.5FFAC–7.5DO hybrid composites had better flexural properties than 5FFAC–5DO and 7.5FFAC–2.5DO due to particle compatibility with the polymer matrix and polymer chain entanglement density. For hybrid composites, 2.5FFAC–7.5DO showed improved storage (<i>E</i>′) and loss moduli (<i>E</i>″). The transition from glassy to rubbery state was delayed by adding FFAC/DO to the polymer matrix, which raised <i>T</i>_g. Cole–Cole plot showed a multiphase in the polymer matrix due to its elliptical shape. Degree of entanglement N and the C factor determined from the storage modulus highlighted filler-matrix relationship.</p> Graphical abstract <p></p>