Fluoride (F–) pollution in groundwater of the Older Alluvial Plain (OAP) of Delhi has been reported as a major problem. About 34% of the groundwater samples collected for this study had F– level beyond the permissible limit; with F– concentration in the range of 0.14–3.15 mg/L (average 1.20 mg/L). In this context, this article for the first time attempts on the genesis of major ions chemistry and F– pollution in groundwater of OAP Delhi by going beyond the statistical analysis to sediment geochemistry, chemical weathering processes and understanding of the processes using stable environmental isotopes (2H and 18O). The XRD of the OAP sediments revealed the dominance of fluor-biotite, albite, calcite, quartz, and chlorite. Whereas, the separated clay revealed the dominance of chlorite, kaolinite, and illite minerals. The saturation index (SI) values indicated that the groundwater chemistry is in the process of further F– enrichment by way of sediment groundwater interaction. With the given mineralogy of the sediments, the dominance of major ions like Na+, K+, Mg2+, Ca2+, Cl– and F– has been attributed to chemical weathering of biotites, phlogopites, albite, and calcite during sediment–water interaction. While the dominance of SO42– has been attributed to anthropogenic sources and confirmed by its association with heavier stable isotopes of hydrogen (δ2H: −50.44 to −40.02‰) and oxygen (δ18O: −7.19 to −5.62‰) indicating evaporative enrichment during isotopic fractionation.