We present a joint computational and experimental study of Mg-Ni-MOF-74 and Mg-Cd-MOF-74 to gain insight into the mixing of metals and understand how metal mixing affects the structure of the undercoordinated open-metal sites. Our calculations predict that metal mixing is energetically preferred in these materials. Recent experimental work has demonstrated that Mg-Ni-MOF-74 shows a much greater surface area retention in the presence of water than Mg-MOF-74. To probe this effect, we study H2O adsorption in Mg-Ni-MOF-74, finding that the adsorption energetics and electronic structure do not change significantly at the metal sites when compared to Mg-MOF-74 and Ni-MOF-74, respectively. We conclude that the increased stability of Mg-Ni-MOF-74 is a result of a M-O bond length distortion in mixed-metal MOF-74, consistent with recent work on the stability of MOF-74 under water exposure.