Abstract: Geopolymer is one new type of cementing material made from aluminosilicate solid wastes, such as metakaolin. The geopolymer is also characterized by a low shrinkage rate, low density, excellent fixation of heavy metals, high resistance to fire, and chemical resistance. However, the adhesive is often prone to stress concentration at the wood-resin bonding interface, thus leading to adhesive cracking and fragile sensitivity to the micro-cracks, even catastrophic damage. This study aims to develop a one-step mechanochemical protocol for the preparation of wood adhesives with the nanocellulose-reinforced geopolymer. The performance of wood bonding was also enhanced by the interfacial bonding strength. Some highly polymeric substances in the MK were transformed into low-polymeric substances and monomer structures (active silicon and aluminum monomers). The active sites on the surface were promoted after mechanochemical preparation. The nanocellulose was also integrated into the geopolymer matrix. Ball milling was employed to generate the nanocellulose from the bamboo pulp fibers. Then the metakaolin was combined to form the geopolymer adhesives. Sodium hydroxide was utilized to mix with the metakaolin and bamboo pulp fiber, in order to adjust the modulus of sodium silicate. The resulting adhesives were designated as the GP-X%CNF, where X represented the percentage of bamboo pulp fiber after addition. Microscopic morphology of the adhesives was then characterized to assess the chemical structure and functional groups using scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR). A series of tests were carried out on the compression, bonding, and flexural strength to evaluate the mechanical properties, according to the national standard of GB/T 9846-2015. Additionally, the thermal stability was detected using thermogravimetry (TG) and differential thermal analysis (DTA). Experimental results showed that the ball-milled nanocellulose significantly enhanced the toughness of the geopolymer-based adhesives, effectively inhibiting the formation and propagation of microcracks. The compressive, bonding, and flexural strength of the plywood samples initially increased and then decreased after the addition of bamboo pulp fibers. The GP-2%CNF samples exhibited optimal performance. The peak values were achieved in all three aspects: a compressive strength of 12.85 MPa, a bonding strength of 2.052 MPa, and a flexural strength of 101.99 MPa. The compressive strength of GP-2%CNF geopolymer increased by approximately 103.65% after 7 days of curing, compared with the GP. The bonding and flexural strength of plywood samples prepared with the adhesive increased by 34.9% and 21.88%, respectively. Among them, the mechanical properties and bonding performance of the geopolymer with 2% cellulose were superior to those with 1% and 4% cellulose. The critical balance was then required in the proportion of nanocellulose for the optimal enhancement of adhesive properties. The dispersion of nanocellulose within the geopolymer matrix was realized to bridge the microcracks, thus improving the overall toughness and durability of the adhesives. Microstructural analysis and FTIR revealed that the chemical structure also depended mainly on the effective dispersion of nanocellulose within the geopolymer matrix. Compression, bonding, and flexural strength tests further validated the contribution rates of the nanocellulose to the performance of geopolymer-based adhesives. Thermal stability analysis indicated that better thermal stability was achieved in the inorganic matrix to effectively protect the organic carbon chains, although the addition of cellulose increased thermal loss. In conclusion, the mechanochemical approach improved the bonding strength at the wood interfaces and the mechanical properties of the wood adhesives with the nanocellulose-toughened geopolymer. An efficient, simple, and environmentally friendly strategy was offered for the rapid and efficient preparation of the organic/inorganic hybrid adhesives. This composite material can be expected to serve as an environmentally friendly, low-cost, and high-temperature-resistant building material, due mainly to the abundant and easily processed resources, such as cellulose and metakaolin.