The phase behavior in blends of magnesium salts of long-chain fatty acids with magnesium-neutralized ethylene-based ionomers is described. Anhydrous magnesium stearate and magnesium stearate-palmitate, collectively termed MgSt(Pm), show a common phase behavior, consisting of a crystalline lamellar mesophase (LAM) bilayer phase at low temperatures, a liquid crystalline hexagonal (HEX) phase at intermediate temperatures, and a molten disordered (DIS) phase at high temperatures. Once heated into the HEX or DIS phase, neat MgSt(Pm) is kinetically prevented from reforming the LAM phase upon cooling. Blending an ionomer into MgSt(Pm) destablizes the HEX phase in favor of DIS; an intimately mixed DIS phase is formed above the LAM and ionomer melting points, where the ionomer acts as an effective solvent for the MgSt(Pm). Quenching this solution forms co-crystals that incorporate alkyl tails from the fatty acid salt and ethylene sequences from the ionomer. Heating the blend to below the LAM melting point melts the co-crystals and allows pure, well-ordered LAM MgSt(Pm) to crystallize from the solution. A generalized phase diagram for MgSt blended with ionomers is presented to describe the observed behavior.