The Atlantic water inflow into the Nordic Seas has proven difficult to reconstruct for the Last Glacial Maximum (LGM). At that time, the Fennoscandian Ice Sheet grew potentially to its maximum extent. Sea-ice free conditions in the eastern Nordic Seas have been proposed as an essential moisture source contributing to this build-up. It has been hypothesized that the inflow of warm and saline Atlantic surface waters was important for maintaining these seasonally sea-ice free conditions in the Nordic Seas at that time. However, the difference between a perennially frozen ocean and a seasonally open ocean on ice sheet build-up remains unquantified.   Here we use, tephra-constrained surface ventilation ages from a network of marine sediment cores and model experiments, to show that Atlantic inflow to the southern Nordic Seas likely occurred predominately via the Iceland-Faroe Atlantic inflow pathway helping to maintain seasonal open waters at the onset of the LGM. Using a numerical snow model, we further demonstrate that such open-ocean conditions may have been a factor contributing to the Fennoscandian Ice Sheet growth with a ~150% increase in surface mass balance over Norwegian coastal areas, compared to sea-ice covered conditions.