Here, we have studied the colloidal properties of carboxyl-amine functionalized superparamagnetic iron oxide nanoparticles (SPIONs with high saturation magnetization) based ferrofluids and their heating efficacies in magnetic fluid hyperthermia (MFH) via specific absorption rate (SAR)/intrinsic loss power (ILP). Moreover, we have systematically investigated the impact of the following heat influencing factors in MFH: (i) concentrations (0.5–8 mg/ml), (ii) surface coatings (trimesic acid (TMA), pyromellitic acid (PMA), terephthalic acid (TA) and aminoterephthalic acid (ATA)), (iii) applied alternating magnetic fields (AMFs - with amplitudes (H)/frequencies (f)) - chosen near to Hergt's biological safety limit, and (iv) dispersion media (biological/non-biological) for using SPIONs in in vitro cancer hyperthermia therapy. SPIONs (particularly decorated with dual-surfactants, i.e., TA-ATA) based aqueous ferrofluids have displayed excellent time-dependent temperature rise even at lower concentrations for the applied AMFs, which resulted in enhanced SAR values ranging from 12.5–200.1 W/g_Fe because of their high colloidal stability and enhanced π-π conjugations from the close structural orientations of TA/ATA molecules due to high electrostatic attractions of the respective functional groups (-COOH/NH₂). Moreover, high ILP values of up-to 3.9 nHm²/kg (higher than the best commercial ferrofluids) are attained on exposure to magnetic fields below the safety limit. Besides, TA-ATA coated SPIONs dispersed in biological/non-biological media have exhibited better thermal responses as compared to their aqueous counterpart and reached therapeutic temperatures at faster rates due to prominent Neel relaxation mechanisms. The highest SAR value of 276.3 W/g_Fe is recorded for TA-ATA coated SPIONs dispersed in triethylene glycol (TEG - with high viscosity), ascribed to the lesser inter-particle interactions from electrostatic repulsions of negative charges among carboxyl/oxygen molecules from SPIONs/TEG respectively. Moreover, TA-ATA coated SPIONs have induced almost 90% cell death in MCF-7 cancer cells in in vitro MFH studies. Thus, the TA-ATA coated SPIONs based ferrofluids have great potential for in vivo/clinical MFH cancer therapies.