<p>More than 60% of transmission lines in China are located in mountainous regions. Therefore, transmission tower foundations experience substantial combined uplift and lateral loads. Challenges faced by existing foundations for transmission towers in mountainous areas include inadequate cost-effectiveness, lengthy construction periods, and significant impacts on the surrounding environment. This study proposes a new type of assembled cable-anchored platform foundation that offers technical advantages in terms of economic viability and environmental preservation. In situ experimental and theoretical investigations were performed to assess the feasibility of this new foundation and determine its mechanical characteristics. Two in situ experiments were conducted to evaluate the bearing capacity of the foundation at a typical mountainous site. Experimental curves showing load–displacement characteristics, anchor cable axial-force variations and subgrade earth-pressure variations were obtained. Subsequently, a theoretical force-balance model was established to evaluate the mechanical performance of the foundation under different load conditions. The in situ experimental results imply that this new type of foundation provides sufficient uplift and lateral bearing capacity. The ultimate failure of the foundation was attributed to the inadequate bearing capacity of precast reinforced concrete components. Theoretical investigations revealed that the composition of the lateral resistance in the foundation depends on the uplift load. These findings provide a reference for the design and application of assembled cable-anchored platform foundations in transmission towers.</p>