We present high quality long-slit spectra of the extended narrow line region (ENLR) near the nucleus of NGC 4151. We have used these data to map the spatial variations of the emission line intensities along three slit positions: one passing radially through the nucleus along the main NE-SW axis of the ENLR and two across this axis at offsets center on bright knots 6 sec and 11 sec to the SW, respectively. We find that, with the exception of some isolated H II regions, all the emitting regions samples by our spectra have line intensities characteristic of photoionization by an active galactic nuclei AGN-like continuum source. Along the radial axis, there is a clear discontinuity at the boundary between the nuclear narrow line region and the ENLR, where a factor approximately 10 drop in gas density coincides with a corresponding increase in the ionization level. Beyond this point there is an overal decline in ionization along the ENLR, while the gas density remains constant. The radial ionization structure is thus broadly consistent with geometrical dilution of a centrally located ionizing continuum source, presumably the active galactic nucleus. Taken together with the kinematics and morphology, these results strongly support the hypothesis that the ENLR arises from photoionization of gas in the galactic disk by an anisotropic radiation field produced by the active nucleus. It is also difficult to reconcile with the morphology of the nuclear narrow line region as revealed by Hubble Space Telescope (HST) images, if this is due solely to anisotropic illumination by the radiation field. However, it is more likely that the configuration of the emitting gas in the nucleus is the product of interactions with the radio ejecta.