  We develop a theory for graphene magnetotransport in the presence of carrier
spin polarization as induced, for example, by the application of an in-plane
magnetic field ($B$) parallel to the 2D graphene layer. We predict a negative
magnetoresistance $\sigma \propto B^2$ for intrinsic graphene, but for
extrinsic graphene we find a non-monotonic magnetoresistance which is positive
at lower magnetic fields (below the full spin-polarization) and negative at
very high fields (above the full spin-polarization). The conductivity of the
minority spin band $(-)$ electrons does not vanish as the minority carrier
density ($n_-$) goes to zero. The residual conductivity of $(-)$ electrons at
$n_- = 0$ is unique to graphene. We discuss experimental implications of our
theory.
