The thermal stability, glass-forming ability (GFA), mechanical properties and Poisson’s ratio of a Zr-based Zr75−xNi25Alx (x = 0 to 24) glassy alloy series have been investigated systematically. The large supercooled liquid region (ΔTx) of over 80 K and critical sample diameter (dc) for glass formation above 10 mm were obtained in a composition range of x = 11 to 21. The maximum dc of 15 mm was formed for an off-eutectic Zr60Ni25Al15 alloy, which had the largest ΔTx and γ value, along with the lowest density change upon crystallization. The bulk glassy alloys exhibited high compressive yield strength (σc,y) of 1662 to 2060 MPa and distinct plastic strain (εc,p) of 1.2 to 4.7%. With increasing Al content, the σc,y and Young’s modulus linearly increased, while the εc,p gradually decreased. The εc,p was found to have a good correlation with the Poisson’s ratio.