Hierarchically porous biocompatible Mg–Al–Cl-type layered double hydroxide (LDH) composites containing aluminum hydroxide (Alhy) have been prepared using a phase-separation process. The sol–gel synthesis allows for the hierarchical pores of the LDH–Alhy composites to be tuned, leading to a high specific solid surface area per unit volume available for high-molecular-weight protein adsorptions. A linear relationship between the effective surface area, SEFF, and loading capacity of a model protein, bovine serum albumin (BSA), is established following successful control of the structure of the LDH–Alhy composite. The threshold of the mean pore diameter, Dpm, above which BSA is effectively adsorbed on the surface of LDH–Alhy composites, is deduced as 20 nm. In particular, LDH–Alhy composite aerogels obtained via supercritical drying exhibit an extremely high capacity for protein loading (996 mg/g) as a result of a large mean mesopore diameter (>30 nm). The protein loading on LDH–Alhy is >14 times that of a reference LDH material (70 mg/g) prepared via a standard procedure. Importantly, BSA molecules pre-adsorbed on porous composites were successfully released on soaking in ionic solutions (HPO42– and Cl– aqueous). The superior capability of the biocompatible LDH materials for loading, encapsulation, and releasing large quantities of proteins was clearly demonstrated.