The evolution of the magnetic ground state of ultrathin 0-10 unit cells (uc) thick La_0.8Sr_0.2MnO₃ films interfaced to an antiferromagnetic La_0.45Sr_0.55MnO₃/SrTiO₃(001) buffer layer was investigated with x-ray photoemission electron microscopy. For 0-3 uc La_0.8Sr_0.2MnO₃, we observe antiferromagnetic domains but no ferromagnetic contrast, showing that nominally ferromagnetic La_0.8Sr_0.2MnO₃ adopts the antiferromagnetic ground state of the buffer layer. For larger thicknesses, ferromagnetic domains emerge, confirming that the additional layers revert to the ferromagnetic ground state. We also observe a drastic increase in the complexity of the domain configuration between 3 and 5 uc, which we attribute to competing magnetic and electronic ground states in the system. We attribute the interfacial modified magnetic ground state to charge sharing at the interface due to the chemical potential mismatch, which leads to hole doping at the La_0.8Sr_0.2MnO₃ interface. The present work sheds light on the impact of charge sharing at the interface of complex oxide materials, in particular on the magnetic and electronic states, and presents a strategy for modulating the electronic ground state properties at metallic interfaces.