The surface of Mars preserves a record of more than 200 paleolakes that are breached and drained by outlet canyons. Here we present observations of outlet breach and canyon geometry for a subset of these basins (n = 24). We show that the volume of water drained during breach formation is a significant predictor of breach depth and cross-sectional area, as well as the volume of material excavated from the outlet canyon. Based on these observations, we conclude that the studied outlet canyons formed during highly erosive, single episodes of lake overflow flooding, and that this mechanism was important for other breached paleolake basins on Mars. Given the large size of many paleolake outlet canyons on Mars, we hypothesize that lake overflow flooding was an important process for shaping the early martian landscape. Finally, we show that outlets of breached lake basins on Earth and Mars follow similar geometric scaling relationships with respect to the potential energy released during the overflow flood. This points to the first-order control of basin size and the physics of lake overflow flooding on the resultant outlet geometry and erosion, regardless of variable boundary conditions (e.g., lithology).