Electric field control of the structure, dimensionality, and reactivity of gold nanoclusters (Au*(20)) deposited on MgO films of various thicknesses supported on Ag(100) are introduced and studied using first-principles electronic structure calculations. Field-controlled interfacial charging and field-induced dimensionality crossover are predicted. For a field E(z)=1 V/nm, an optimal planar Au(20) island on MgO(8 layers)/Ag(100) is determined, while for E(z)=0, the preferred structure of the cluster is a tetrahedron. Field control of the reactivity of the adsorbed nanocluster with O(2) is discussed.