Polycrystalline samples of MgB₂ doped with reactive nanostructured carbon were synthesized by pressure assisted sintering of mechanically alloyed precursors. Varying the nominal carbon concentration from x = 0 to 0.316, the effects of carbon doping on the lattice parameter, lattice strain, actual amount of incorporated carbon (x_actual), grain size, normal state resistivity (ρ), connectivity, superconducting transition (T_c), critical fields (B_irr and B_c2) and critical current density (J_c) as well as the pinning force (F_p) were evaluated. An evident solubility limit of carbon within the MgB₂ matrix, forming MgB_2−xC_x with an x_actual≈0.125, was observed. In addition to the carbon saturation the superconducting properties, e.g. T_c, B_c2 and J_c, also reflect saturation effects with respect to the actual carbon concentration. Improved electron scattering in MgB_2−xC_x seems responsible for the observed enhancement of B_c2 to 11.4 T at 20 K. On the other hand, calculations of the flux-pinning forces show a dramatic decrease of F_p,max with increasing carbon concentration. Therefore we conclude the observed improvement in critical current density at applied fields >6 T to result mainly from the raised upper critical field.