The considerable effect of workmanship quality on the performance of masonry structures is well-known. In this study, a 3D nonlinear FE model was used to investigate the effect of workmanship quality on masonry wall strength under in-plane and out-of-plane loads. Mortar joint cohesion was utilized as a random variable to represent variation in workmanship quality. In-plane and out-of-plane displacements were applied to the wall, separately and simultaneously. Results indicate that mortar cohesion has a considerable effect on wall strength under in-plane loads. However, under combined loads, the influence of workmanship quality on wall strength decreases/increases for the in-plane/out-of-plane direction.