The structures of two mixed-ligand [Mg(Q)(thd)2] complexes (thd = 2,2,6,6-tetramethylheptane-3,5-dionate ion (dipivaloylmethanate ion), Q = propane-1,3-diamine (pda), N,N,N′,N′-tetramethylpropane-1,3-diamine (tmpda)) are determined by X-ray crystallography. The structure of the complex with tmpda is determined at 296 K, that of the complex with pda is determined at 296 K and 150 K. Crystallographic data (296 K) for [Mg(pda)(thd)2] I are: space group P(Formula presented.), a = 9.6363(3) Å, b = 12.1063(4) Å, c = 14.3401(5) Å, α = 70.6430(10)°, β = 72.7680(10)°, γ = 89.376(2)°, V = 1500.41(9) Å3, Z = 2. Crystallographic data for [Mg(tmpda)(thd)2] II are: space group С2/с, a = 10.3716(3) Å, b = 17.3614(4) Å, c = 19.1577(5) Å, β = 90.8480(10)°, V = 3449.26(16) Å3, Z = 6. Crystal packings are formed by mononuclear molecules that are organized in dimers by N–H…O hydrogen bonds in the structure of I. Magnesium atoms have a distorted octahedral coordination environment. In I and II molecules, Mg–O bond lengths are comparable and vary in ranges 2.027-2.062 Å and 2.023-2.030 Å respectively; OMgO chelate angles are close to 86°. In complex II, Mg–N bond lengths and the NMgN angle are remarkably larger than those in I: 2.331 vs. 2.193-2.229 Å; 89.6 vs. 84.7°. A comparative analysis of molecular structures and crystal packings of related [Mg(Q)(thd)2] complexes with diamines of propylene and ethylene series is carried out.