Through the in situ decomposition of DMSO (dimethyl sulfoxide), heterometallic inorganic frameworks, formulated as [KLn(SO₄)₂H₂O] (Ln = Sm 1, Eu 2, Gd 3, Tb 4, Dy 5, Ho 6, Er 7) were synthesized under the help of facile and effective one-pot solvothermal reactions. These in situ reactions enhanced the reactivity and coordination ability of the sulfate anions, leading to the formation of new 3D lanthanum sulfate frameworks with optical and magnetic properties. Compounds 2 and 4 are strong luminescent materials which display characteristic Eu³⁺ and Tb³⁺ emission bands in the visible region with high quantum yields and long luminescent lifetimes. Variable-temperature magnetic susceptibility studies revealed that 3 and 5–7 show anti-ferromagnetic behaviors. Their thermal stability, demonstrated from the TGA profiles and temperature dependent XRD measurements, is among the highest of reported lanthanum sulfate frameworks. Additionally, the band structure calculations for 3, and 7, based on the density functional theory methods, were also performed.