It was envisioned that rendering the surface charge of cotton fabric cationic at acidic pH would allow it dyeable with natural dyes. For this purpose and with a vision for introducing a completely green process, the fabric was pretreated with tannic acid followed by padding in gelatin aqueous solution and curing. The factors that may affect this pretreatment process were investigated, as reflected by the color strength obtained of the dyed fabrics using madder dye. Furthermore, the pretreated fabric (bio-mordanted) was characterized by FTIR to confirm the success of chemical modification, and a tentative mechanism for the dye fixation is believed to be physical bonds (hydrogen bond and van der Waals bond) and chemical bond (ionic bond). The selected conditions obtained were comparatively applied using four natural dyes, namely, madder (CI Natural Red 9), curcumin (CI Natural Yellow 3), rhubarb (CI Natural Yellow 23), and alkanet (CI Natural Red 20). The successful results presented in this work and the excellent fastness properties obtained suggest its potential as a viable and ecological method for successful bio-mordanting and dyeing of cotton fabrics with natural dyes.