  Given an $n$-dimensional manifold $N$ with an affine connection $D$, we show
that the associated Patterson-Walker metric $g$ on $T^*N$ admits a global and
explicit Fefferman-Graham ambient metric. This provides a new and large class
of conformal structures which are generically not conformally Einstein but for
which the ambient metric exists to all orders and can be realized in a natural
and explicit way. In particular, it follows that Patterson-Walker metrics have
vanishing Fefferman-Graham obstruction tensors. As an application of the
concrete ambient metric realization we show in addition that Patterson-Walker
metrics have vanishing Q-curvature.
