A first‐principles description of the magnetic properties of the antiferromagnetic alloys Rb2Mnp Ni 1−pF4, Rb2Mnp Mg 1−pF4, and RbMnp Mg 1−pF3, based on detailed calculations, is presented. The theory gives a good account of the existing neutron scattering data, as well as the temperature dependence of the sublattice magnetization and the nonlinear static susceptibility, and makes a number of predictions for future experiment. The calculations provide microscopic details of fluctuation phenomena due to disorder in antiferromagnets which do not occur in ferro‐ or ferri‐magnets.