The accurate definition of the mechanical properties of masonry is challenging, even in the case of regular assemblies. Various sources of uncertainty stem from craftsmanship techniques, block shape and texture, properties of individual components (mortar and bricks/stones), environmental conditions, prior damage, deterioration, etc. These factors contribute to a wide variability in the mechanical properties of masonry, necessitating the consideration of uncertainties for reliable predictions of the ultimate capacity of masonry buildings. A significant challenge is the limited availability of experimental data, which hinders a reliable probabilistic characterization of uncertain mechanical properties. To address this issue, the present study adopts a non-probabilistic approach known as the interval model. The key idea is to represent the uncertain parameters as interval variables with specified lower bound and upper bound. The study focuses on the in-plane behavior of unreinforced masonry walls. A benchmark masonry wall, extensively documented in the literature, is analyzed using a Finite Element (FE) model based on a simplified micro-modeling approach. Selected mechanical properties are then modeled as interval variables and incorporated into the FE model to explore the impact of uncertainties on the wall’s failure modes and ultimate capacity. Uncertainty propagation is performed by applying a ratio of polynomial Response Surface. Numerical results are presented to demonstrate the accuracy of the Response Surface approach and to assess the influence of uncertain mechanical properties on the in-plane behavior of the masonry wall.