MgAl₂O₄ transparent ceramics possess excellent optical property, but their practical applications are somewhat restricted by the hydrolysis problem during the shaping process and limited mechanical property. Meanwhile, it has been demonstrated that the property of quaternary MgAlON spinel can be effectively adjusted via varying their composition. Accordingly, a novel Mg_0.9Al_2.08O_3.97N_0.03 transparent ceramic with a broad transmittance range was prepared by combining aqueous gel-casting, pressureless sintering, and hot isostatic pressing treatment. Optical and mechanical property of this transparent ceramic were systematically investigated and compared with those of MgAl₂O₄ transparent ceramic. Furthermore, the slow crack growth under low stress were analyzed, and the service life of transparent ceramic was predicted. It is shown that the viscosity of ceramic slurry with 50% (in volume) solid load was 124 mPa·s, which could meet the requirement of aqueous gel-casting. The in-line transmittance of 86.2% at 3.7 μm was obtained in the Mg_0.9Al_2.08O_3.97N_0.03 transparent ceramic sample with thickness of 2 mm, and the optical transmittance range was comparable to that of MgAl₂O₄, with slightly higher refractive index and Abbé number. Further, this ceramic showed a Weibull modulus similar to MgAl₂O₄, and although its crack slow growth coefficient is lower than MgAl₂O₄, but both the characteristic strength (210.6 MPa) and inert strength (227.5 MPa) were much higher. Therefore, in the quaternary MgAlON spinel with low nitrogen content, the hydrolysis problem of ceramic powders could be well overcome, while the mechanical property of transparent ceramic was remarkably improved without degradation of optical property. This research provides a new pathway toward obtaining the novel spinel transparent ceramics with improved preparation and property.