We report magnetic, transport and neutron diffraction studies of the solid solution Mn1−xCoxSi. For the Mn rich compounds, a sharp decrease of the Curie temperature is observed upon cobalt doping and neutron elastic scattering shows that the helimagnetic order of MnSi persists up to x = 0.06 with a shortening of the helix period. For higher Co concentrations (0.06 < x < 0.90), the Weiss temperature changes sign and the system enters an antiferromagnetic state upon cooling (TN=9K for x = 0.50). In this doping range, the antiferromagnetic coupling leads to a Kondo effect marked by a minimum in the resistivity. This scenario is supported by the scaling of the magnetoresistance with a TK ≈ 6.5 K, close to the change in curvature of the resistivity and in agreement with the Weiss temperature from magnetic susceptibility. The sign change of the Weiss temperature and the transition from a helimagnetic to an antiferromagnetic ground state, with increasing the Co doping, point toward the existence of a quantum critical point at the composition Mn0.94Co0.06Si.