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Animal cell shape changes are controlled by the actomyosin cortex, a peripheral actin network tethered to the plasma membrane by membrane-to-cortex attachment (MCA) proteins. Previous studies have focused on how myosin motors or actin turnover can generate the local deformations required for morphogenesis. However, how the cell controls local actin nucleation remains poorly understood. By combining molecular engineering with biophysical approaches and in situ characterization of cortical actin network architecture, we show that membrane-to-cortex tethering determines the distance between the plasma membrane and the actomyosin cortex at the nanoscale of single actin nucleators. In turn, the size of this gap dictates actin filament production and the mechanical properties of the cell surface. Specifically, it tunes formin activity, controlling actin bundling and cortical tension. Our study defines the membrane-to-cortex distance as a nanogate that cells can open or close by MCA proteins to control the activity of key molecules at the cell surface.