In strongly correlated oxides, heterostructures provide a powerful route to manipulate the charge, spin, orbital, and lattice degrees of freedom to create distinctive functionalities. In this work, we have achieved atomically precise interface control in YBa₂Cu₃O_{7 x}/La_0.7Ca_0.3MnO₃ (YBCO/LCMO) heterostructures and find a hidden effective doping. This mechanism is responsible for higher T_c in the sample with the MnO₂-terminated interface than in that with the La_0.7Ca_0.3O-terminated interface. The MnO₂-terminated sample also shows a larger magnetic moment of Mn together with a lower valence state. For more than a decade, the control of T_c in these heterostructures prior to this work has been solely via the variation of YBCO or LCMO thickness. This work hints at an alternative way of exploiting and exploring the interactions between superconductivity and magnetism in this system.