A new oxide, LaMn3Co2Mn2O12, was synthesized under high-pressure (7 GPa) and high-temperature (1423 K) conditions. The compound crystallizes in an AA'3B4O12-type quadruple perovskite structure with space group Im (3) over bar. The Rietveld structural analysis combined with soft X-ray absorption spectroscopy reveals the charge combination to be LaMn33+Co22+Mn24+O12, where the La3+ and Mn3+ are 1:3 ordered respectively at the A and A' sites, whereas the Co2+ and Mn4+ are disorderly distributed at the B site. This is in sharp contrast to R2Co2+Mn4+O6 (R = La and rare earth) double perovskites, in which the Co2+ and Mn4+ charge states are always orderly distributed with a rocksalt-type fashion, giving rise to a long-range magnetic ordering. As a result, LaMn3Co2Mn2O12 displays spin glassy magnetic properties due to the random Co2+ and Mn4+ distribution, as demonstrated by dc and ac magnetic susceptibility as well as specific heat measurements. Possible factors that affect the B-site degree of order in perovskite structures are discussed.