We use first-principles theory to predict that the application of uniaxial compressive strain leads to a transition from an antiferromagnetic insulator to a ferromagnetic half-metal phase in LaMnO_3. We identify the Q2 Jahn-Teller mode as the primary mechanism that drives the transition, indicating that this mode can be used to tune the lattice, charge, and spin coupling. Applying ≃ 6 GPa of uniaxial pressure along the [010] direction activates the transition to a half-metallic pseudo-cubic

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... e. The half-metallicity opens the possibility of producing colossal magnetoresistance in the stoichiometric LaMnO_3 compound at significantly lower pressure compared to recently observed investigations using hydrostatic pressure.