This study reports on the exceptional stability of cobalt‐mediated dye‐sensitized solar cells (DSSCs) assembled in a monolithic configuration with a carbon counter electrode (CE). DSSCs using [Co(bpy)3]2+/3+ redox mediators often face stability challenges due to light–induced interaction with tert‐butylpyridine (TBP), an electrolyte additive. This leads to the loss of electrochemically active mediators and degradation of the photovoltaic performance. However, the monolithic DSSCs in this work demonstrate unprecedented stability under continuous light soaking for over 1000 h. The stability is attributed to the device architecture and the carbon CE, which mitigate critical degradation mechanisms. Additionally, its adsorption capacity and opacity reduce the amount of free TBP in the electrolyte, thereby suppressing harmful photoinduced [Co(bpy)3]³+‐TBP complexation and water‐induced redox reactions. Despite high TBP concentrations (1.2 M), which typically accelerate degradation in conventional DSSCs, the monolithic devices maintained their performance due to electrolyte retention and reduced ion diffusion within the porous carbon layer. This study highlights the critical role of architecture in stabilizing cobalt‐mediated DSSCs, paving the way for robust, long‐term energy conversion applications.