Availability of magnesium is a matter of concern due to its role in many environmental and biological processes. Diffusive Gradients in Thin Films (DGT) devices can measure Mg availability in situ. This work shows that Mg accumulation in water largely increases when ionic strength (I) decreases. This phenomenon can be explained from (i) the increase of both the association equilibrium (K) and rate (ka,R) constants for the reaction between Mg cations and resin sites, and (ii) the growing contribution of the partitioning of Mg cations at the resin–gel interface, as I decreases. Two theoretical models that take into account electrical interactions among Mg cations, background electrolyte, and resin sites can successfully be used to determine ka,R and K at each I. Both models yield similar ka,R values, which fulfill an expression for the kinetic salt effect. For freshwater (with a typical salinity of 10 mM and circumneutral pH), the binding of Mg is so fast and strong that the simplest perfect-sink DGT expression can be helpful to predict (overestimation lower than 5%) the accumulation in solutions with Mg concentrations up to 1 mM whenever the deployment time is below 9 h. Perfect sink conditions can still be applied for longer times, in systems with either a lower I or a lower Mg concentration.