Stratigraphy and its associated grain size preserve a record of the dynamic behaviour of source‐to‐sink systems over time. Sediment supply and available accommodation space primarily control downstream grain‐size fining preserved in stratigraphy. In principle, these grain‐size trends can be inverted to quantify temporal and spatial variation in these driving forces. Here, we illustrate how grain size and stratigraphic thickness can be used to quantify fault growth and interaction using the early‐mid Pleistocene Pirgaki‐Mermoussia (P‐M) fault, Gulf of Corinth, Greece, as a natural laboratory. A 2.5 km long exposed cliff section of the uplifted Kerinitis Gilbert‐type delta, which lies in the hanging wall of the P‐M fault, was selected for study. In the field, we traced out stratigraphic units in the lower part of the Kerintis delta, which are bounded by flooding surfaces, and measured their thickness to reconstruct hanging wall subsidence. We collected down‐system grain‐size data at 31 measurement sites using the Wolman point count method. Our results show the observed grain‐size fining rate increase from 11 to 17 mm.km ⁻¹ for the lower delta deposits over a timescale of up to 120 kyr. Using a self‐similarity‐based grain‐size fining model and considering a minimum increase in accommodation generation from 0.6 to 1 mm year ⁻¹ over this period, we reconstruct an increase in delta sediment supply from ca. 170 to 460 m ³ year ⁻¹ . The integration of stratigraphic thickness measurements with grain‐size fining trends enables quantitative reconstruction of temporal variations in fault‐driven accommodation space and sediment supply, thereby demonstrating fault slip rate evolution. We show an increase in the P‐M fault slip rate during its early history from 1 to 2 mm year ⁻¹ , reflecting early interaction of the P‐M fault segments over ca. 120 kyr. Reconstructed catchment‐averaged erosion rates are ca. 20% of the footwall uplift, implying a transient response of the landscape to the P‐M fault growth. These analyses demonstrate how grain‐size data from a well‐constrained geological example can be used to reconstruct landscape dynamics quantitatively in fault‐controlled sedimentary systems with high temporal and spatial resolution.