Lignin containing cellulose nanofibril (LCNF) was reported, for the first time, to improve the performance of polymeric diphenylmethane diisocyanate (pMDI) as a wood adhesive. The LCNF aqueous suspension was either solvent-exchanged (LCNF/SE) or freeze-dried (LCNF/FD) before mixing with pMDI at three loading amounts. The addition of 3 wt% LCNF/SE and 3 wt% LCNF/FD to pMDI improved the dry strength by 109% and 69% and the wet strength by 119% and 84%, respectively. The hydroxyl groups of LCNF reacted with the isocyanate groups of pMDI and formed polyurethane linkages. The curing kinetics study indicated that the activation energy of curing pMDI with LCNF was higher than that of curing pMDI with wood (wood contained 10% moisture). The morphological observations showed that the wood joints using neat pMDI had an inconsistent and partly starved bondline. LCNF dramatically improved the adhesive gap-filling properties by serving as a reactive medium to bridge the gaps and helped to form a continuous bondline. The sustainable LCNF usage could allow pMDI to become a more versatile wood adhesive with broader applications.