We report the synthesis, structural investigation, and magnetic property studies of Sr1.31Co0.63Mn0.37O3 that adopts an incommensurate composite hexagonal perovskite-related structure. The crystal structure has been solved using a (3 + 1)-dimensional superspace approach from powder X-ray and neutron diffraction data (SSG R3̄m(00γ)0s, a = 9.5548(1) Å, c = 2.5599(1) Å, q = 0.65581(4)c*, AB = 0.041, Rp = 0.059). The structure consists of face-sharing chains of octahedra and trigonal prisms, wherein the trigonal prismatic sites are preferentially occupied by Co with some cation disorder. A combination of electron diffraction and high-resolution electron microscopic analysis has demonstrated that the compound possesses a complicated microstructure related to the formation of domains with slightly different lengths of the modulation vector. X-ray absorption near-edge spectroscopic (XAS) studies clearly indicate the presence of Mn in the 4+ and Co in the 3+ oxidation state. While the magnetic susceptibility data indicates the presence of antiferromagnetic correlations in the system, the calculation of effective paramagnetic moment (μcal = 3.561 μB), assuming the metal oxidation states as obtained from XAS and the cation distribution as obtained from neutron refinement, is in agreement with the value obtained experimentally (μexp = 3.676 μB).