The lateral performance of structurally nonsymmetric diagonal-braced wood shear walls used in post-and-beam timber buildings was investigated with laboratory testing and computer modeling. Monotonic and reversed cyclic tests were performed to study the capacities of a total of eight full-scale shear walls, with and without gypsum wallboard sheathing. Significant increases of strength, stiffness, and energy dissipation were observed in the shear walls with the additional gypsum wallboard sheathing. An efficient mechanics-based wood shear wall model was then introduced to represent the hysteretic load–drift behavior of these diagonal-braced walls. The predicted seismic response of the model agreed very well with the shake table test results of a simple one-story post-and-beam structure, making it possible to develop, in future research, structural models for more-complicated post-and-beam buildings with good computational efficiency and accuracy.