Mangiferin modified membranes were prepared based on amorphous poly(amide) (PA)/poly(vinyl pyrrolidone) (PVP) blends by coagulation via solvent (dimethyl sulfoxide, DMSO) and non-solvent (water) exchange. To understand miscibility behavior and provide guidance to kinetic pathways of the coagulation process, various ternary phase diagrams of PA/DMSO/water, PA:PVP/DMSO/water, and mangiferin/DMSO/water mixtures were constructed. The ternary phase diagram of PA/DMSO/water mixture is consisted of isotropic liquid and liquid+liquid coexistence regions, whereas the mangiferin/DMSO/water system exhibited a crystal+isotropic coexistence region. Addition of PVP to the PA/DMSO/water solution showed little or no effect on the ternary phase diagram of PA:PVP/DMSO/water, whereas the addition of mangiferin to PA/DMSO/water resulted in enlargement of the liquid+liquid immiscibility gap. The actual amounts of mangiferin in the final membranes were quantified in relation to the mangiferin in feed. The surface and cross-sectional gradient morphologies of the mangiferin modified and unmodified membranes were examined using scanning electron microscopy (SEM) and the surface porosity was found to increase with PVP loading. A theoretical phase diagram for the simplest case of polymer/solvent/non-solvent system was calculated self-consistently in the framework of Flory–Huggins free energy for liquid–liquid demixing and compared with the experimental phase diagram of the PA/DMSO/water mixture. Finally, a prism phase diagram was presented to qualitatively illustrate a coagulation pathway of the mangiferin modified PA:PVP blend membrane through the solvent/non-solvent exchange followed by drying.