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Abstract Microfossils can have a large impact on the hydromechanical properties of marine sediments. Here, we study how these properties change in sediment mixtures containing varying concentrations of diatoms during experimental loading. We mixed an illite‐rich glaciomarine clay known as Boston Blue Clay (BBC) and a smectite‐rich marine clay known as Eugene Island Clay (EI) with marine and lacustrine diatoms in mass ratios of 100:00, 90:10, and 80:20. These mixtures were uniaxially compressed to 100 kPa in resedimentation tests and further loaded to 2 MPa in constant rate of strain consolidation experiments. We found that adding diatoms results in an increase in void ratio, compressibility, and vertical permeability at a given vertical effective stress for both sediments. These changes are due to an increased intraskeletal and interskeletal porosity caused by the porous nature of diatoms and their ability to form stress bridges. With increasing vertical effective stress, sediments lose their permeability at a slower rate when containing diatoms. These changes are most evident in BBC mixtures. When comparing both sediment types, void ratio and permeability decrease faster during burial for the EI mixtures than the BBC mixtures. These results provide new insights into the hydromechanical behavior of microfossil‐rich marine sediments and contribute to our understanding of their potential for overpressure generation and the development of a weak layer.