Co-substituted manganese oxide nanosheets, (Mn1xCox)O2 have been synthesized in the form of a colloidal suspension via a simple one-pot method. Substitution effects on the structural, optical absorption, and magnetic properties are investigated for the nanosheets and their lamellar aggregates. The composition of the (Mn1xCox)O2 nanosheets can be controlled continuously by adjusting the molar ratio of the starting materials. The solubility limit is x0.20 based on the cell volume. In the 0.00 x 0.20 range, the band gap energy, magnetic moment, and Weiss temperature change systematically with x. The charge density of the (Mn,Co)O2 layer is independent of x (i.e., [(Mn,Co)O2]0.2) and the cobalt ions are trivalent in low-spin state