In this work, synergistic impact of hybrid fibers and hybrid fillers on mechanical, moisture absorption and morphological properties of polyester matrix composites at dry and wet conditions was examined. The hybrid composite laminates were fabricated by compression molding technique, using new combinations of jute and coir fibers as primary reinforcements as well as waste eggshell powder (ESP) and montmorillonite nanoclay (MMTK10-NC) as secondary reinforcements in a polyester matrix. The fillers, ESP and NC were added to the jute and coir fiber blended hybrid composite in different amounts. Maximum tensile strengths of the hybrid composites were estimated to be 24.57, 21.77 and 22.63 MPa when fillers were added at equal weight percent (wt.%) of 1.5 for dry, river and sea water (wet) conditions, respectively. Under same dry, river and sea water conditions, flexural strengths of 46.97, 44.85 and 42.36 MPa as well as impact strengths of 2.697, 2.572 and 2.607 J were obtained, respectively. Occurrence of swelling, agglomeration and porosity on the hybrid composites were observed to be crucial factors, which caused their strength reduction. It was evident from the results obtained that addition of hybrid fillers to the hybrid composites in different weight proportions increased their mechanical performances higher than that of non-filler hybrid composites. Mechanical properties diminished with wet condition, due to deterioration between the fibers and matrix interface. Also, scanning electron microscopy (SEM) images indicated that it was due to weak fiber-matrix adhesion. With addition of hybrid filler to the hybrid composites, the moisture absorption rate was reduced. Importantly, the optimal/best hybrid composite sample S4 with equal filler content of 1.5 wt% can be used as an alternative sustainable material for low load bearing structural components in building construction industries, such as trusses and frames, wooden flooring and scaffold.