It is important to determine if the dynamics of mixed-clay sediment gravity flows can be predicted from their dominant clay type, because natural gravity flow deposits can contain mixtures of clay minerals of different cohesive strength, and latitudinal zonation in clay mineral production may influence depositional patterns in mud-rich submarine fans. The present lock-exchange experiments produced high-density sediment gravity flows carrying different proportions of strongly cohesive bentonite clay and weakly cohesive kaolinite clay with a fixed 20% volumetric concentration. Head velocity and run-out distance of the flows reduced, and starting suspension yield stress increased, as bentonite fraction increased beyond 20%. However, for bentonite fractions ≤ 20%, the initial suspensions had lower yield stresses and the flows were more mobile than the pure kaolinite flow, implying small bentonite fractions reduce the cohesive strength of the suspensions. Predictive equations for mixed-clay suspension yield stress, and head velocity and run-out distance of mixed-clay flows, based on yield stresses of pure-clay constituents, indicates minimal interaction between the constituents for bentonite fractions ≤ 20% and increasingly non-linear interactions for bentonite fractions > 20%. These results suggest natural sediment gravity flow dynamics and deposits may be sensitive to the most cohesive clay rather than the dominant.