The effect of nanoscale-SiC doping of MgB2 was investigated in comparison with undoped, clean-limit, and Mg-vapor-exposed samples using transport and magnetic measurements. It was found that there are two distinguishable but related mechanisms that control the critical current-density-field JcsHd behavior: increase of upper critical field Hc2 and improvement of flux pinning. There is a clear correlation between the critical temperature Tc, the resistivity r, the residual resistivity ratio RRR=Rs300 Kd /Rs40 Kd, the irreversibility field H*, and the alloying state in the samples. The Hc2 is about the same within the measured field range for both the Mg-vapor-treated and the SiC-doped samples. However, the JcsHd for the latter is higher than the former in a high-field regime by an order of magnitude. Mg vapor treatment induced intrinsic scattering and contributed to an increase in Hc2. SiC doping, on the other hand, introduced many nanoscale precipitates and disorder at B and Mg sites, provoking an increase of rs40 Kd from 1 mV cm sRRR=15d for the clean-limit sample to 300 mV cm sRRR=1.75d for the SiC-doped sample, leading to significant enhancement of both Hc2 and H* with only a minor effect on Tc. Electron energy-loss spectroscope and transmission electron microscope analysis revealed impurity phases: Mg2Si, MgO, MgB4, BOx, Six ByOz, and BC at a scale below 10 nm and an extensive domain structure of 2–4-nm domains in the doped sample, which serve as strong pinning centers.