The fields of electronics and spintronics require an active control and manipulation of the charge and spin degrees of freedom [1].Valleytronics, on the other band, is very new field that relies on the property that the conduction/valence bands have two or more minima/maxima at equal energy but different momenta.For valleytronics devices it is necessary to induce valley polarization, i.e., control the number of electrons in each valley, typically by strain [2] or a magnetic field [3, 4].We propose in this work a method to control the valley polarization by magnetic doping.Various possible Mn doping sites in single layer MoS2 are studied to investigate the influence of the exchange field on the electronic structure by first-principles calculations.We will argue that the strength of the spin orbit coupling together with the exchange energy determine the valley polarization, which can be inverted by modifying the spin polarization.