The title ionic compound, (C21H27N2)[AuCl4]·CH2Cl2, was obtained from the reaction of 1,3-dimesitylimidazolidinium chloride with t-BuOK and a solution of AuCl3 in tetra-hydro-furan. In the crystal structure, numerous weak C - H⋯Cl hydrogen bonds form double layers parallel to (100), which are further stabilized by π-π inter-actions between mesitylene rings [centroid-centroid distance = 4.308 (4) Å], resulting in the formation of a three-dimensional supra-molecular assembly.