The storage and release of sediment from glacierised catchments is an important process in mountain landscape evolution, and yet sediment transport pathways and residence times within glaciers remain poorly constrained. We quantified headwall erosion rates and englacial sediment transport and storage times in the Mer de Glace catchment in the Mont Blanc massif, French Alps, during deglaciation through the Holocene (11.7 ka to present). Englacial sediment transport and storage times were constrained using luminescence rock surface burial ages of granitic clasts sampled along the central flow line of the Mer de Glace ablation area. We also used luminescence rock surface exposure dating and terrestrial cosmogenic nuclide (¹⁰Be) measurements to constrain headwall erosion rates for this catchment. These headwall erosion and sediment transport data were compared with simulated erosion, sediment trajectories and transport rates derived from the glacier model iSOSIA. Measured headwall erosion rates were ~0.1–5 mm a⁻¹ and are consistent with other estimates from the Mont Blanc massif. Luminescence rock surface burial ages ranged from ~0.6 to ~6.7 ka and clustered into distinct age populations at ~0.8 ka, ~1.5 ka, ~2.2 ka, and ~6.7 ka. The youngest age population is consistent with continuous englacial transport times predicted by the glacier modelling and observations of present-day glacier surface velocity, whereas the older age clusters indicate prolonged sediment storage within the catchment. Comparison of our results with results from Miage Glacier, Italian Alps, shows that long-term sediment storage with durations exceeding 1 ka is common in steep alpine glacierised catchments, despite high erosion rates and active ice flow. Luminescence burial ages indicate that sediment can be stored during periods of glacier minima, then released during more active phases. Glacierised catchments therefore act as millennial-scale sediment reservoirs, introducing time lags between sediment production and downstream transport, that modulate climatic signals recorded in proglacial stratigraphy during deglaciation.