The recycling of waste printed circuit boards (WPCBs) generates nonmetallic fractions (NMFs); due to the complex components of NMFs and the limited nature of economic benefits of treating NMFs, treatment of NMFs is challenging. In this study, two types of NMFs—dry-NMFs (D-NMFs) and wet-NMFs (W-NMFs)—derived from the dry and wet separation processes of WPCBs, respectively, were investigated. These NMFs were used as fillers to reinforce the polyethylene (PE) matrix, and their effects on the composite properties were examined. Thermal property studies revealed that incorporating both types of NMFs improved the thermal stability of the prepared composite samples. When neat PE was filled with 15 wt% of D-NMFs and W-NMFs, the final decomposition temperature (T_f) increased from 475 to 482 and 487 ℃, respectively. Mechanical property studies revealed that the addition of NMFs to the composite sample, particularly that of W-NMFs, enhanced the stiffness of the prepared samples, although at the expense of some reduction in their toughness values. The tensile strength, tensile modulus, flexural strength, and flexural modulus values increased from 9.41, 121.80, 5.89, and 99.15 MPa for neat PE to 11.15, 521.82, 17.94, and 597.29 MPa, respectively, for composites containing 25 wt% of W-NMFs. Furthermore, the introduction of shellfish wastes in the NMF/PE blend, especially that of clam shell, further enhanced the overall properties of the composite. After adding 8 wt% of clam shell with 15 wt% W-NMFs, the T_f increased from 487 to 498 ℃. The tensile strength, tensile modulus, flexural strength, and flexural modulus values increased from 11.37, 355.13, 16.06, and 443.31 MPa for neat PE to 12.26, 466.73, 18.71, and 568.46 MPa, respectively, for the composite prepared with clam shell. Thus, this study contributes to the WPCB recycling literature and promotes circular economy development.