The Mn substitution effect on superconductivity and spin correlations in La1.90Sr0.10Cu1−y Mn y O4 was investigated by means of magnetic susceptibility and neutron scattering measurements. The superconducting (SC) transition temperature (T c) and the SC volume fraction are reduced by Mn substitution with y = 0.01. It was clarified that the peak-splitting of the incommensurate (IC) low-energy spin excitations narrows by 30% and the magnetic ordering temperature is significantly enhanced in the y = 0.01 sample. Thus, the SC state and the magnetic ordering state coexist at low temperature. Furthermore, incommensurability (δ), corresponding to half the distance between the IC peaks, and T c (∼ 18 K) of y = 0.01, both follow the linear T c−δ relation established for the pristine La2−x Sr x CuO4 (LSCO). Therefore, although Tc varies as a result of Mn substitution, spin correlations are closely coupled to superconductivity as is the case for LSCO.