The physical and mechanical properties of 12 glasses from the Y-(Mg,Ca)-Si-Al-O-N and (Mg,Ca)-Si-Al-O-N systems were investigated. The effect of the substitution of magnesium for calcium through two series of glasses, one consisting of oxides glasses and the other of glasses containing 6 at.% of nitrogen (15 e/o N), was considered. The change of the glass transition temperature through the glass series provides evidence for a mixed-alkaline-earth effect between magnesium and calcium species. The indentation hardness (H), Young's modulus (E), and indentation fracture toughness (K-C) were found to increase significantly with either the magnesium or the nitrogen content, and nitrogen also seems to enhance the effect of magnesium on the properties. The network structure was analyzed both by Si-29 and Al-27 Magic Angle Spinning Nuclear Magnetic Resonance and by neutron scattering experiment, which allows for the estimation of some atomic bond lengths in such complex glasses. Nitrogen was found to have a significant structural effect on the magnesium environment and on the glass polymerization degree, and hence on the glass properties.