Strontium-doped lanthanum manganite (LSM) is one of the most widely used cathode material for yttria-stabilized zirconia (YSZ)-based solid oxide fuel cells (SOFCs), due to its superior chemical stability and small coefficient of thermal expansion (TEC) mismatch with YSZ. The activation process of the cell, which can be attributed to the cathode and cathode electrode interface, has a significant effect on the long-term electrochemical performance and stability of SOFCs. Therefore, it is essential to understand the origin of this underlying activation process. In this study, thin film LSM on YSZ half cells were fabricated with an LSCF counterelectrode and the effects of cathodic current density and overpotential on the polarization behavior of the LSM thin film electrode were investigated. The surface morphology as well as surface chemistry and bonding environment of LSM before and after polarization was investigated using scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. The formation of SrO was found on the LSM surface after polarization. The cross-section microstructure of LSM, as well as interfacial chemistry of LSM/YSZ, were investigated using transmission electron microscope (TEM) and TEM-based energy dispersive X-ray spectroscopy (EDS). The presence of Mn-rich phases were observed before and after polarization. The pre- and post-activation electrode performances were investigated at various temperatures and oxygen partial pressures, which helps reveal the nature of the activation mechanism in combination with the microstructural and chemical analysis.