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 (xactual), grain size, normal state resistivity (), connectivity, superconducting transition (Tc), critical fields (Birr and Bc2) and critical current density (Jc) as well as the pinning force (Fp) were evaluated. An evident solubility limit of carbon within the MgB2 matrix, forming MgB2-xCx with an xactual≈0.125, was observed. In addition to the carbon saturation the superconducting properties, e.g. Tc, Bc2 and Jc, also reflect saturation effects with respect to the actual carbon concentration. Improved electron scattering in MgB2-xCx seems responsible for the observed enhancement of Bc2 to 11.4 T at 20 K. On the other hand, calculations of the flux-pinning forces show a dramatic decrease of Fp,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.