The low-temperature thermodynamic properties and magnetic structure of the garnet oxide {Mn${}_3$V${}_2$Ge${}_3$O${}_{12}$} (MVGO) have been investigated via magnetic susceptibility, specific heat, and neutron diffraction measurements. MVGO exhibits two successive magnetic transitions at $T_{\rm N1} = 4$ K and $T_{\rm N2} = 2.6$ K, associated with the antiferromagnetic ordering of noncollinear Mn${}^{2+}$ and collinear V${}^{3+}$ sublattices, respectively. The Curie-Weiss analysis based on the susceptibility data and the magnetic entropy derived from the specific heat measurement confirmed the ground state $S=5/2$ for Mn${}^{2+}$ and $S=1$ for V${}^{3+}$. Density functional theory calculations reproduce the experimentally determined magnetic structures, yielding nearest-neighbor antiferromagnetic couplings $J_{Mn-Mn}$ and $J_{V-V}$ along with a ferromagnetic coupling $J_{Mn-V}$. By applying a magnetic field, we construct a temperature–field phase diagram for MVGO that encompasses multiple distinct phases. These results establish MVGO as a complex system among garnet-structure magnets.