The fabrication of monoliths by environment-friendly methods without organic solvents remains a challenging task. In this study, we successfully produced mesoporous polymer monoliths by selective hydrolysis degradation of miscible poly(-L-lactic acid) (PLLA)/poly(methyl methacrylate) (PMMA) blends without any organic solvents. 50 to 90 wt% of PLLA blend ratios were tested. We found that freeze-dried was effective to prevent a shrinkage of monoliths. With increaser of PLLA weight fraction, porosity, mode pore diameter, and pore geometry could be controlled from 17 to 52%, 7.1–32 nm, and mainly cylindrical to spherical, respectively. Further, a comprehensive model based on volume balance and surface area equivalence successfully predicted mode pore size. The reported organic solvent-free fabrication of mesoporous polymer monoliths might enable tunable mesopore properties through only changing mechanical blending of miscible polymers.