Fox nutshell, also known as Makhana, is an agricultural residue found in tropical and subtropical regions. Likewise, black cotton (BC) soil is troublesome and primarily spread in similar environmental conditions. However, the effect of fox nutshell ash (FNA) amendment on the BC soil’s strength and water retention properties is missing in the previous literature. Thus, this study aims to overcome this research gap. Therefore, BC soil and FNA were procured from the local trader and ground with a mechanical grinder in laboratory. Thereafter, BC soil was mixed with an increasing proportion (0, 5, 8, and 10%) of FNA and specific gravity, Atterberg’s limits, compaction, unconfined compressive strength (UCS) and water retention capacity (WRC) tests were carried out in the laboratory following the ASTM standards. The results showed that with the addition of 10% FNA to BC soil, the specific gravity decreased by 8.4%, while the liquid limit increased by 4.81%, and the plastic limit increased by 2.67%. The optimum moisture content increased from 27.51–30.75%, and maximum dry density decreased from 1.535–1.365 g/cc with the inclusion of 10% FNA in the BC soil. The UCS value of 5% FNA amended soil increased by 7.28% after a period of curing of 28 days, indicating an increase in the soil’s strength. Moreover, the WRC was also increased by 5% for a 10% FNA addition. The increased WRC and strength of ash-amended BC soil suggest that it could be used as landfill cover material. The findings of this research could be insightful for engineers and researchers dealing with ash-amended BC soil for geotechnical and geo-environmental engineering projects.