After their emplacement in the course of the late Mesozoic and the Cenozoic, the Hellenic nappes became fragmented during late orogenic extension since the late Eocene. Here we focus on the transition of underthrusting during nappe emplacement to exhumation during late orogenic extension. To this end, we compared previously published data on the structural geological and metamorphic history of the underthrusted parts of the Tripolitza and Ionian nappes, which were exhumed in the Cycladic and South Aegean windows, with newly obtained data on the sedimentary, stratigraphic, and structural development of the part of these nappes in the foreland, in front of the subduction thrust. The results allow the identification of two major events: Event 1 took place around the Eocene-Oligocene transition and marks the onset of underthrusting of the Tripolitza nappe below the Pindos nappe and the Ionian nappe below the Tripolitza nappe, respectively. This led to the uplift and erosion of the Pindos unit and the onset of deposition of the Tripolitza and Ionian flysch in front of the Pindos thrust, together with the formation of mylonites at the base of the metamorphosed portions of the Pindos unit related to the underthrusting of the Tripolitza unit. Event 2 occurred in the latest Oligocene to earliest Miocene and marks the decoupling of the Ionian unit from the underthrusting plate, the accretion of the Tripolitza and Ionian units to the overriding plate, and the onset of late orogenic extension and exhumation in the overriding plate. This led to the formation of the South Aegean and Cycladic core complexes and the subsidence of the Klematia-Paramythia half-graben throughout the early Miocene.