<p>Access to clean and safe drinking water is essential for improving the health ladder towards prosperity and sustainable well being. However, decades-old drinking water distribution systems (DWDSs) pose significant challenges in maintaining drinking water quality in the line of the established standards in many developing countries, including Pakistan. This study assessed the water quality and corrosion scale of decades-old DWDSs in District Mardan (northern Pakistan) focusing on both intermittently operated tube well (TW) and continuously operated spring water (SW) systems. Standards methods were employed to perform different physico-chemical water quality tests, corrosion scale characterizations and microbiological investigation. Characterization results revealed that the corrosion scales contained about 18% iron and 37% carbon. Interestingly, dense layers of α-Fe₃O₄, γ-Fe₂O₃ and FeO were the more prominent compositions. Water quality assessment results revealed significant variations between the water quality of intermittently and continouesly operated DWDSs, but all the parameters were within the World Health Organization (WHO) permissible limits. Water quality index (WQI) of twenty collected water samples revealed that the spring water samples are of better quality, with most being rated as excellent or good. In contrast, the majority of tube well water samples are rated as very poor or unfit for drinking. Microbiological investigations indicated the presence of both Gram-negative and Gram-positive bacteria in the corrosion scales, with <i>Enterococcus</i> and <i>Bacillus</i> identified as the dominant species. The findings from this study underscore the importance of regular water quality monitoring and periodically replacement of iron pipes to address public health concerns in such decades-old DWDSs.</p>