While working for Shaw, Lethaby became involved in the Society for the Protection of Ancient Buildings, which campaigned to preserve the integrity and authenticity of older buildings against the Victorian practice of 'improving' them to the point of almost completely rebuilding and redesigning them. Through this he became a personal friend of Arts and Crafts Movement pioneers William Morris and Philip Webb, becoming a significant and influential member of their circle and acting as co-founder of the Art Workers Guild in 1884. He was a lifelong socialist.