  The acute sensitivity of the electrical resistance of certain systems to
magnetic fields known as extreme magnetoresistance (XMR) has recently been
explored in a new materials context with topological semimetals. Exemplified by
WTe$_{2}$ and rare earth monopnictide La(Sb,Bi), these systems tend to be
non-magnetic, nearly compensated semimetals and represent a platform for large
magnetoresistance driven by intrinsic electronic structure. Here we explore
electronic transport in magnetic members of the latter family of semimetals and
find that XMR is strongly modulated by magnetic order. In particular, CeSb
exhibits XMR in excess of $1.6 \times 10^{6}$ % at fields of 9 T while the
magnetoresistance itself is non-monotonic across the various magnetic phases
and shows a transition from negative magnetoresistance to XMR with field above
magnetic ordering temperature $T_{N}$. The magnitude of the XMR is larger than
in other rare earth monopnictides including the non-magnetic members and
follows an non-saturating power law to fields above 30 T. We show that the
overall response can be understood as the modulation of conductivity by the Ce
orbital state and for intermediate temperatures can be characterized by an
effective medium model. Comparison to the orbitally quenched compound GdBi
supports the correlation of XMR with the onset of magnetic ordering and
compensation and highlights the unique combination of orbital inversion and
type-I magnetic ordering in CeSb in determining its large response. These
findings suggest a paradigm for magneto-orbital control of XMR and are relevant
to the understanding of rare earth-based correlated topological materials.
