Tumour protein p53 (TP53) is a tumour suppressor gene that is frequently mutated in cancers. Traditional TP53 detection methods, such as polymerase chain reactions, are time-consuming and demand skilled laboratory personnel. As an alternative, in the current study, we have demonstrated a high resistivity silicon (HR-Si) based conductometric biosensor designed for the rapid and specific identification of TP53 point mutations directly at the point-of-need. This biosensor accurately detected R248Q and R248W point mutant single strand DNA (ssDNA) as models, in real-time. Both R248Q and R248W mutant ssDNA exhibited a limit of detection (LOD) of 0.5 ng/mL in human plasma. The selectivity studies revealed that both R248Q and R248W mutant ssDNA can be detected 10 × lower molar content against their wild-type ssDNA. Validation of the sensor using clinical samples harbouring known TP53 mutations demonstrated a sensitivity of 100%, a specificity of 100%, and a LOD of 2.5 ng/mL. This precision biosensing platform at the point-of-need has the potential to revolutionise cancer diagnostics.