Interactions within mixtures of the phenothiazine drug promethazine hydrochloride (PMH) and cationic hydrotropes ortho-toluidine hydrochloride (o-TDH) and para-toluidine hydrochloride (p-TDH) were investigated at different ratios and temperatures via conductometry to understand various physicochemical properties. Critical micelle concentration (cmc) was less than values of cmcid (cmc in ideal mixed system), indicating significant interaction among the studied constituents in solution mixtures. The cmc of pure PMH was also determined by measuring the surface tension for comparison. A variety of micellization thermodynamic parameters (Gibbs free energy [ΔGm0], change in standard enthalpy [ΔGm0], and change in entropy [ΔGm0]) were computed using conductometry. The micellar mole fraction (X1Rb, X1Rod, and X1id) of hydrotropes estimated by various theoretical models (Rubingh, Rodenas, and Motomura) was assessed, and the results showed a greater contribution of hydrotropes in mixed micelles along with their values increasing via an increase in mole fraction (α1) of hydrotropes (o-TDH/p-TDH). Negative β values suggest extremely favorable attractive interaction/synergism, as declines occurred in the whole quantity of amphiphile used for the desired purpose, leading to a drop of expenditure along with ecological concern. Obtained activity coefficients (f1 and f2) were always beneath unity, meaning nonideality was found between PMH and o-TDH/p-TDH. Like the conductivity method, the UV-visible and FT-IR techniques also demonstrate the interaction between the PMH and o-TDH/p-TDH.