We report a photoelectron imaging study of the pyridinide anion, C5H4N-, obtained by deprotonation of pyridine at the C4 position. The photoelectron angular distributions are used to test the theoretical formalism for photodetachment from mixed-character s-p states, demonstrating its first application to a truly polyatomic system. The mixed s-p model describes the initial state of the anion in terms of a superposition of one s orbital and one p orbital centered on the deprotonated carbon. Using the model parameter values determined from ab initio calculations, without any fits to the experimental data, the theory yields good quantitative agreement to the experiment. The agreement is demonstrated using either the canonical Hartree-Fock highest-occupied molecular orbital of the anion or the corresponding Dyson orbital. The results confirm the predictive power of the mixed s-p model and suggest that despite its approximate nature it captures the essential physics of the photoemission process.