  Neutron powder diffraction (NPD) study of \textit{Ln}MnSbO (\textit{Ln }$=$
La or Ce) reveals differences between the magnetic ground state of the two
compounds due to the strong Ce-Mn coupling compared to La-Mn. The two compounds
adopt the \textit{P4/nmm} space group down to 2 K and whereas magnetization
measurements do not show obvious anomaly at high temperatures, NPD reveals a
C-type antiferromagnetic (AFM) order below $T_{\mathrm{N}} = 255 $ K for
LaMnSbO and 240 K for CeMnSbO. While the magnetic structure of LaMnSbO is
preserved to base temperature, a sharp transition at $T_{\mathrm{SR}} = 4.5 $K
is observed in CeMnSbO due to a spin-reorientation (SR) transition of the
Mn$^{\mathrm{2+}}$ magnetic moments from pointing along the $c$-axis to the
\textit{ab}-plane. The SR transition in CeMnSbO is accompanied by a
simultaneous long-range AFM ordering of the Ce moments which indicates that the
Mn SR transition is driven by the Ce-Mn coupling. The ordered moments are found
to be somewhat smaller than those expected for Mn$^{\mathrm{2+}}$ ($S = 5/2$)
in insulators, but large enough to suggest that these compounds belong to the
class of local-moment antiferromagnets. The lower $T_{\mathrm{N\thinspace }}$
found in these two compounds compared to the As-based counterparts
($T_{\mathrm{N}} = 317$ for LaMnAsO, $T_{\mathrm{N}} = 347$ K for CeMnAsO)
indicates that the Mn-$Pn$ ($Pn=$ As or Sb) hybridization that mediates the
superexchange Mn-$Pn$-Mn coupling is weaker for the Sb-based compounds.
