Quantifying the velocity, volume, and rheology of deep, slow‐moving landslides is essential for hazard prediction and understanding landscape evolution, but existing field‐based methods are difficult or impossible to implement at remote sites. Here we present a novel and widely applicable method for constraining landslide 3‐D deformation and thickness by inverting surface change data from repeat stereo imagery. Our analysis of La Clapière, an ~1 km2 bedrock landslide, reveals a concave‐up failure surface with considerable roughness over length scales of tens of meters. Calibrating the thickness model with independent, local thickness measurements, we find a maximum thickness of 163 m and a rheology consistent with distributed deformation of the highly fractured landslide material, rather than sliding of an intact, rigid block. The technique is generally applicable to any mass movements that can be monitored by active or historic remote sensing. Key Points We invert landslide velocity and elevation change data for the 3D slip surface La Clapiere landslide has a maximum thickness of 163m and volume of 38million m3 Distributed deformation, rather than block sliding, best fits observations