Mesoscale eddies play a central role in the oceanic energy cycle by mediating energy exchanges between large-scale circulation and small-scale dissipative processes. However, how cross-scale energy transfer evolves throughout the mesoscale eddy life cycle and what mechanisms regulate the associated energy cascade remain poorly understood. Based on satellite altimetry and the high-resolution numerical simulation, this study investigates bidirectional kinetic energy transfer among large-scale, mesoscale, and submesoscale motions within a normalized eddy life-cycle framework. Conventional altimetry shows weak inverse transfer from mesoscales to larger scales, whereas the high-resolution simulation resolves submesoscale ageostrophic motions and reveals stronger bidirectional transfer dominated by a forward cascade toward submesoscales. Lifecycle composites show that cyclonic eddies gain energy from large-scale motions during generation, whereas anticyclonic eddies exhibit weak or negative LS–MS transfer. MS–SM transfer remains generally positive and strengthens during decay, highlighting eddy decay as a key stage for transferring mesoscale energy toward submesoscales. This enhanced forward transfer is closely linked to submesoscale frontogenesis: positive frontogenesis is associated with enhanced forward energy transfer and accounts for approximately 80% of the total forward mesoscale-to-submesoscale transfer. The strongest frontogenesis and forward cascade both occur during late decay, when eddy deformation and filamentation are most pronounced. These findings reveal a lifecycle-dependent, frontogenesis-driven pathway for mesoscale eddy energy dissipation in the Southern Ocean, with important implications for understanding oceanic energy cascades and improving the representation of mesoscale–submesoscale interactions in climate models.