Sulfur-based glasses have reemerged as promising candidates for use as solid electrolytes in Li-based batteries. Nevertheless, due to their amorphous structure, the ion migration mechanisms that underlie their high Li-ion conductivity remain poorly understood. The present study employs ab initio molecular dynamics to reveal the local structure and migration mechanisms in the prototype Li-ion conducting glass, 0.75 Li₂S- 0.25 P₂S₅. A computational model of the amorphous structure was generated, and is shown to closely match the measured neutron weighted pair distribution. The structure data indicates that Li-ions experience a range of coordination environments, with typical Li-S coordination numbers between 3 to 5. Lithium is observed to migrate via correlated, 'string-like' events involving multiple adjacent cations. Furthermore, these migration events involve the dynamic participation of the PS₄anions, which undergo simultaneous rotation and translational displacements. This behavior contrasts with that of the Li₃PS₄ crystalline analogue, were rotations and translations of the anions during migration events are severely limited. These observations provide direct evidence of the importance of anion dynamics on cation mobility in fast ion conductors.