Compacted bentonite/sand mixtures are recognized as optional sealing/backfilling materials in the deep geological repository for long-lived high-level radioactive wastes worldwide. In this context, it is important to guarantee that the gas production after the repository closure does not jeopardize the barrier system. This study investigates the hydro-mechanical-gas transfer behavior of compacted bentonite/sand mixtures by experimental and numerical approaches. The compression behavior during static compaction for sample preparation was firstly analyzed. A factor defined as the contribution of bentonite to the overall compressibility was introduced to clarify the formation of sand skeleton at low bentonite fractions. Then, the swelling pressure, hydraulic conductivity and gas injection tests were conducted under constant-volume condition. Besides, the local heterogeneity of compacted mixtures was determined experimentally. The effects of bentonite fraction, dry density, sand nature and heterogeneity on the hydro-mechanical-gas transfer behavior was analyzed. The dominant mechanism of gas transfer after gas breakthrough was determined. The materials satisfying the Andra’s requirements for a sealing core were defined. Furthermore, the hydro-mechanical behavior upon suction-controlled oedometer loading was investigated. Finally, a numerical modeling focusing on the mixture heterogeneity was conducted to analyze the gas transfer. An original method was proposed to describe the development of gas preferential pathways.