Investigations of the elastic mechanical characteristics of loosely compacted mixed sand-clay samples are essential for the characterization of shallow seafloor sedimentary reservoirs. Here, we used a series of sand-clay mixtures (0–100% clay by weight) to investigate the effects of pressure load histories (2–50 MPa) and clay fraction on porosity, P-wave velocity, S-wave velocity, and static modulus. The results indicated that, under pressure load cycles, the sand-clay mixture samples exhibited high permanent compaction, and porosity decreased rapidly with increasing clay content. However, porosity changed differently in response to pressure load as compared to pressure unload. Correspondingly, P-wave velocities differed significantly between pressure load and unload. The rock physical models implied that the sample frame evolved with the varying clay fraction. As the clay fraction increased, the sample frame gradually transitioned from primarily sand grains with dispersed pore-filling clay to primarily clay, ultimately transforming into a clay frame with dispersed sand grains. The measured data generally conformed to the theoretical model and falling within the Voigt and Reuss bounds. Moreover, for all sand-clay mixture samples, although the static bulk modulus was consistently lower than the dynamic bulk modulus, the ratio of the dynamic modulus to the static modulus was highly dependent on pressure loads and clay fraction.