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Face-centred cubic (fcc-) Mo N and Ta N exhibit an inherent driving force for vacancy formation. To study their interaction and effects on structural evolution and mechanical properties, we synthesised Mo–Ta–N coatings by reactive magnetron sputtering using nitrogen-to-total pressure ratios, pN2/pT, of 0.32 and 0.69. Low pN2/pT results in high concentration of N vacancies, which stabilise single-phase fcc-Mo1-xTaxNy up to x = 0.76. These solid solutions follow the MoN0.5–Ta0.875N0.875 quasi-binary tie line. Compressive residual stresses, σ, indentation hardness, H, and toughness, KC, increase with Ta content, reaching their maxima of (on average) -2.0 GPa, 28 GPa, and 7.0 MPa√m, respectively, within the x range 0.38–0.69. Higher Ta contents favour higher concentration of metal vacancies deteriorating the properties. High pN2/pT favours the formation of fcc-Mo1-xTaxNy rich in metal vacancies, which however always coexists with a hexagonal phase. Within the x range 0.33–0.66, the fraction of the hexagonal phase is negligible, and σ, H, and KC deviate from −1.0 GPa, 28 GPa, and 2.9 MPa√m, respectively, within the error of measurements. The combination of experimental and theoretical studies demonstrates the power of point defects in stabilising desired crystal structures and improving mechanical properties through the thereby tuned atomic configuration.