Abstract Strong-field terahertz (THz) excitations enable dynamic control over electronic, lattice and symmetry degrees of freedom in quantum materials. Here, we uncover pronounced terahertz-induced symmetry modulations and coherent phonon dynamics in the van der Waals antiferromagnet MnPS 3 , in which inversion symmetry is broken by its antiferromagnetic spin configuration. Our time-resolved second harmonic generation measurements reveal long-lived giant oscillations in the antiferromagnetic phase, with amplitudes comparable to the equilibrium signal, driven by phonons involving percent-level atomic displacements relative to the equilibrium bond lengths. The temporal evolution of the rotational anisotropy patterns indicates a dynamic breaking of mirror symmetry, modulated by two vibrational modes at 1.7 THz and 4.5 THz, with the former corresponding to a hidden mode not observed in equilibrium spectroscopy. These effects arise in part from a field-induced charge rearrangement mechanism that lowers the local crystal symmetry and couples to the phonon modes. We uncover a long-lived field-driven response with a complex THz polarization dependence, consistent with a metastable state of reduced symmetry. Through comparison with simulations, we interpret this response in terms of a potential antiferromagnetic-to-ferrimagnetic transition. Our results establish an effective field-tunable pathway for driving excitations otherwise weak in equilibrium.