Two new Mn(III) complexes with unprecedented topologies containing the tert-butylphosphonate ligand (t-BuPO₃²⁻), [Mn₉O₆(t-BuPO₃)₂(O₂CMe)₁₁(MeCOOH)(H₂O)]·8H₂O (1), and [NBuⁿ₄][Mn₁₃O₆(t-BuPO₃)₁₀(OH)₂(N₃)₆(MeCOOH)₂(H₂O)₂]·6H₂O (2) have been prepared by treatment of Mn(O₂CMe)₂ and NBuⁿ₄MnO₄ with tert-butylphosphonic acid in the presence of different bases. The core of 1 consists of two [Mn₃O] units connected to a near linear [Mn₃] unit by four μ₃-O, while the core of 2 possesses a Mn centred disklike [Mn₇O₆] unit linked to six additional manganese atoms via six μ₄-O. Compounds 1 and 2 are both homovalent manganese(III) cage clusters, of which the six-coordinated Mn centers are all Jahn–Teller distorted. Magnetic susceptibility measurements reveal that compounds 1 and 2 display overall ferromagnetic and antiferromagnetic interactions, respectively, between the adjacent Mn^III ions. Both the in-phase signal χ′_MT and out-of-phase signal χ″_M of the two complexes exhibit frequency-dependence below approximately 3 K.