Lightweight Mg alloys are desirable for transportation and defense applications owing to their high specific strength and energy‑absorption capacity under impact loading. Among these, Mg–Zn–Y alloys containing long-period stacking-ordered (LPSO) phases exhibit exceptional comprehensive mechanical properties; however, their deformation behavior and underlying micromechanisms under high-speed shear remain poorly understood. Therefore, this study performs a comparative analysis of the microstructural evolution and dynamic mechanical behavior of as-cast and extruded Mg–Zn–Y–0.3Sn (mass fraction, %) alloys containing LPSO phases using a split Hopkinson pressure bar at strain rates ranging from 800 s⁻¹ to 1800 s⁻¹. The aim is to elucidate the strength response, identify the dominant deformation mechanisms, and clarify how microstructural evolution, including features characteristic of LPSO-containing Mg–Zn–Y alloys, couples with the high-strain-rate shear response. The results show that as the strain rate increases from 800 s⁻¹ to 1800 s⁻¹, the ultimate compressive strength of the extruded Mg–4Zn–12Y alloy increases from 279.16 MPa to 354.78 MPa, showcasing an enhancement of approximately 27.1%. Similarly, increasing the strain rate from 800 s⁻¹ to 1600 s⁻¹ raises the ultimate compressive strength of the extruded Mg–4Zn–12Y–0.3Sn alloy from 342 MPa to 433 MPa, corresponding to an improvement of approximately 26.6%. The Sn addition enhances the alloy’s strength and work-hardening capacity. The {1012} tensile twinning primarily serves to coordinate plastic strain, whereas kinking the LPSO phase plays a critical role in coordinating the deformation. High densities of geometrically necessary dislocations accumulate at grain-boundary triple junctions, producing considerable local lattice distortion and providing favorable conditions for twin nucleation. Furthermore, dynamic recrystallization occurs around the LPSO phase, driven primarily by the particle-stimulated nucleation mechanism and the complex morphology of the LPSO phase/matrix interface. These findings provide crucial insights into the deformation micromechanisms of LPSO-containing Mg alloys under high-strain-rate conditions.