Exquisite control of complex systems including atoms, molecules, and crystal lattices is a recurring theme across the natural sciences. In solids, ultrafast light has emerged as a powerful tool to access hidden phases with exceptional speeds. Yet in most cases, photoinduced phases collapse back to equilibrium once the drive is removed, limiting their utility. Here I introduce a pathway to stabilize and control light‑induced phases by harnessing critical fluctuations. Using strong-field THz excitation in the van der Waals antiferromagnet FePS₃, I generate a metastable magnetized state inaccessible in equilibrium via nonlinear driving of magnon-phonon hybrids. The lifetime of the metastable magnetization diverges around the Néel temperature, reaching an experimental maximum of 2.5 milliseconds. By extracting the critical exponents governing the metastable magnetization amplitude and relaxation time, we develop a comprehensive picture of how fluctuations stabilize the metastable order. More strikingly, I demonstrate coherent tunability of the magnetized state: by controlling the phase of coherent phonon oscillation, I achieve on-demand, bidirectional magnetic switching. These results establish fluctuation‑enabled, ultrafast control of hidden magnetic phases via dynamical spin–lattice interactions, opening routes to next‑generation magnetic devices.