Single crystals of the new transition metal Zintl phase, Ca21Mn4Sb18, were prepared by high temperature melt synthesis. The crystal structure was determined by single crystal X-ray diffraction to be monoclinic in the space group C-2/c. Crystal information was obtained at 90 K, and unit cell parameters were determined (a = 17.100(2) Angstrom, b = 17.073(2) Angstrom, c = 16.857(2) Angstrom, beta = 92.999(2)degrees, Z = 2, R1 = 0.0540, wR2 = 0.1437). The structure can be described as containing 4 discreet units per formula unit: 1 linear [Mn4Sb10](22-) anion, 2 dumbbell-shaped [Sb-2](4-) anions, 4 individual Sb3- anions, and 21 Ca2+ cations. The [Mn4Sb10](22-) anion contains four edge-shared MnSb4 tetrahedra with distances between Mn ions of 3.388(4) Angstrom, 2.782(4) Angstrom, and 2.760(4) Angstrom. Electron counting suggests that the Mn are 2+. Temperature dependent magnetization shows a ferromagnetic-like transition temperature at similar to52 K which is suppressed with increasing magnetic field. The paramagnetic regime is best fit to a ferrimagnetic model, providing a total effective moment of 4.04(2) mu(B), significantly less than that expected for 4 Mn2+ ions (11.8 mu(B)). Temperature dependent resistivity shows that this compound is a semiconductor with an activation energy of 0.159(2) eV (100-300 K).