Quantum-critical many-body states host universal correlations extending across all length scales. Whether these critical correlations merely characterize quantum-critical matter or can actively generate interactions that organize collective order has remained an open question. Here we establish the concept of quantum-critical mediation, in which a quantum-critical state functions as an active mediator that dynamically generates effective interactions between otherwise weakly coupled degrees of freedom. We demonstrate this mechanism experimentally in a radical–Ni quantum magnet, where strongly coupled spin-1/2 chains realize a Tomonaga–Luttinger liquid (TLL) and anisotropic spin-1 moments remain as low-energy degrees of freedom. Combining thermodynamic measurements, quantum Monte Carlo simulations, bosonization, and conformal field theory, we show that critical TLL correlations generate an emergent two-dimensional interaction network that stabilizes long-range antiferromagnetic order. Our results reveal a previously unrecognized function of quantum-critical matter—not merely as a phase exhibiting universal correlations, but as an organizing element that creates interactions and drives collective order. This work establishes quantum-critical mediation as a general organizing principle for emergent collective phenomena and provides a framework for engineering collective states from critical many-body correlations.