We report the observation of fully magnetically polarized ultrathin La_0.8Sr_0.2MnO₃ films by using LaMnO₃ and La_0.45Sr_0.55MnO₃ buffer layers grown epitaxially on SrTiO₃(001) substrates by molecular beam epitaxy. Specifically, we show that La_0.8Sr_0.2MnO₃ films grown on 12-unit-cell LaMnO₃ have bulk-like magnetic moments starting from a single unit cell thickness, while for the 15-unit-cell La_0.45Sr_0.55MnO₃ buffer layer, the La_0.8Sr_0.2MnO₃ transitions from an antiferromagnetic state to a fully spin-polarized ferromagnetic state at 4 unit cells. The magnetic results are confirmed by X-ray magnetic circular dichroism, while linear dichroic measurements carried out for the La_0.8Sr_0.2MnO₃/La_0.45Sr_0.55MnO₃ series show the presence of an orbital reorganization at the transition from the antiferromagnetic to ferromagnetic state corresponding to a change from a preferred in-plane orbital hole occupancy, characteristic of the A-type antiferromagnetic state of La_0.45Sr_0.55MnO₃, to preferentially out of plane. We interpret our findings in terms of the different electronic charge transfers between the adjacent layers, confined to the unit cell in the case of insulating LaMnO₃ and extended to a few unit cells in the case of conducting La_0.45Sr_0.55MnO₃. Our work demonstrates an approach to growing ultrathin mixed-valence manganite films that are fully magnetically polarized from the single unit cell, paving the way to fully exploring the unique electronic properties of this class of strongly correlated oxide materials.