  Lightlike Cartan geometries are introduced as Cartan geometries modelled on
the future lightlike cone in Lorentz-Minkowski spacetime. Then, we provide an
approach to the study of lightlike manifolds from this point of view. It is
stated that every lightlike Cartan geometry on a manifold $N$ provides a
lightlike metric $h$ with radical distribution globally spanned by a vector
field $Z$. For lightlike hypersurfaces of a Lorentz manifold, we give the
condition that characterizes when the pull-back of the Levi-Civita connection
form of the ambient manifold is a lightlike Cartan connection on such
hypersurface. In the particular case that a lightlike hypersurface is properly
totally umbilical, this construction essentially returns the original lightlike
metric. From the intrinsic point of view, starting from a given lightlike
manifold $(N,h)$, we show a method to construct a family of ambient Lorentzian
manifolds that realize $(N,h)$ as a hypersurface. This method is inspired on
the Feffermann-Graham ambient metric construction in conformal geometry and
provides a lightlike Cartan geometry on the original manifold when $(N,h)$ is
generic.
