Clay platelets/cellulose nanofibril nanocomposites are recyclable engineering materials of interest for sustainable development. There is substantial experimental data for mechanical properties, but modeling efforts are scarce. Here, a conceptual unit cell with voids was used in finite element modeling. Predictions for the in-plane modulus taking voids into account were more accurate than the classical rule of mixtures. Simulations also reveal that the cellulosic matrix undergoes shear and tensile deformation with inclined fracture located near the end of platelets for low clay content while predicting brittle tensile failure for high clay content. The results from the unit cell approach provide improved understanding of experimental observations, supporting the strive to better understand mechanisms of deformation and fracture.