  We apply the gradient expansion approximation to the light-cone gauge,
obtaining a separate universe picture at non-linear order in perturbation
theory within this framework. Thereafter, we use it to generalize the $\delta
N$ formalism in terms of light-cone perturbations. As a consistency check, we
demonstrate the conservation of the gauge invariant curvature perturbation on
uniform density hypersurface $\zeta$ at the completely non-linear level. The
approach studied provides a self-consistent framework to connect at non-linear
level quantities from the primordial universe, such as $\zeta$, written in
terms of the light-cone parameters, to late time observables.
