High-resolution $\gamma$-ray spectroscopy of ${}^{18}\mathrm{N}$ is performed with the Advanced GAmma Tracking Array, following deep-inelastic processes induced by an ${}^{18}\mathrm{O}$ beam on a ${}^{181}\mathrm{Ta}$ target. Six states are newly identified, which together with the three known excitations exhaust all negative-parity excited states expected in ${}^{18}\mathrm{N}$ below the neutron threshold. Spin and parities are proposed for all located states on the basis of decay branchings and comparison with large-scale shell-model calculations performed in the p-sd space, with the YSOX interaction. Of particular interest is the location of the $0_1^{-}$ and $1_2^{-}$ excitations, which provide strong constrains for cross-shell p-sd matrix elements based on realistic interactions and help to simultaneously reproduce the ground and first-excited states in ${}^{16}\mathrm{N}$ and ${}^{18}\mathrm{N}$, for the first time. Understanding the ${}^{18}\mathrm{N}$ structure may also have significant impact on neutron-capture cross-section calculations in r-process modeling including light neutron-rich nuclei.