[en] Highlights: • Good mechanical properties was achieved in a novel ultralight Mg-2.76Li–3Al-2.6Zn-0.39Y alloy. • Bimodal microstructures during multidirectional forging (MDF) of this alloy were discovered and elucidated. • Intense mechanically shearing fragmentation and incomplete DRX are deformation mechanism of MDF. • A criterion of judging the occurrence of the Portevin–Le Chatelier effect was proposed and verified. • The contributions of different strengthening mechanisms to tensile yield strength were estimated. A novel Mg-2.76Li–3Al-2.6Zn-0.39Y alloy was prepared by multidirectional forging (MDF) and rolling (R) to improve strength and ductility, and its microstructural evolution and mechanical properties were investigated by an optical microscope, X-ray diffraction (XRD), transmission electron microscope (TEM), and tensile tester. After MDFR, the ultimate tensile strength was 275 ± 8 MPa, the yield strength was 207 ± 7 MPa, and the elongation was 29% at a grain size of 6.8 ± 1.1 μm. The specific modulus of this alloy was 30.04 MN m kg⁻¹, and the specific strength was 183.58 kN m kg⁻¹. Microstructural examination of decreased-temperature MDF revealed the presence of a bimodal microstructure, which is favorable for the enhancement of both strength and ductility. The deformation mechanisms of MDF are intense mechanically shearing fragmentation and incomplete dynamic recrystallization. XRD analysis revealed that the present alloy consists of α(Mg) phase, and Al₂Y, Mg₁₇(Al,Zn)₁₂, Mg₂Y, and AlLi intermetallic compounds. TEM analysis confirmed the existence of Mg₁₇(Al,Zn)₁₂ compound and dislocation wall and pile-up. Portevin–Le Chatelier (PLC) effect or serrated flow was discovered in this alloy. The relationship between the break-away stress and the experimental stress was proposed as a criterion for judging the occurrence of the PLC effect. The estimated stress is consistent with the experimental stress. The contributions of different strengthening mechanisms to tensile yield strength were estimated. The estimated tensile yield strength is in good agreement with the experimental tensile yield strength.