Generating surface defects and vacancies in catalyst supports is a powerful way to enhance activity for Haber-Bosch catalysts. Here, we synthesized a series of Mo-based oxynitride pyrochlores, Ln₂Mo₂O_7‑xN_y☐_z (Ln = Sm, Gd, Ho, ☐_z = vacancy) and explored for ammonia synthesis. The Rietveld refinements of X-ray data and XPS analysis of the synthesized oxynitride pyrochlores show the progressive increase in anion vacancies, z = 0.22, 0.32, to 0.65 for Ho, Gd and Sm, respectively. Ru/Sm₂Mo₂O_3.87N_2.48☐_0.65 gives superior activity compared to Ru/Gd₂Mo₂O_4.19N_2.49☐_0.32 and Ru/Ho₂Mo₂O_4.44N_2.38☐_0.22. The difference in activity was correlated with anion vacancies, and could also be seen for within Sm compositions; between Ru/Sm₂Mo₂O₇, and Ru/Sm₂Mo₂O_2.77N_3.26☐_0.97 a two-fold activity difference was observed (2.2 vs 4.74 mmol∙g^-1∙h^-1) at 400 ˚C at 5 MPa. Based on activity, ¹⁵N₂/¹⁴N₂ isotope equilibrium experiments and kinetics studies, N₂ is activated by a combination of the Ru and anionic vacancies on oxynitride pyrochlore. The stability test with Ru/Sm₂Mo₂O_3.87N_2.48☐_0.65 shows that catalysts are highly stable up to 90 h at 400 ˚C at 5 MPa. This study further expands on the role of nitride and vacancies for ammonia synthesis, on a wide-ranging, air-stable material family.