The world’s growing population has resulted in an increase in the production of garments, which in turn has led to a rise in garment waste. This study aims to investigate the potential use of garment waste fibers (GWF) in concrete production, along with 20% fly ash as a replacement of cement. The effects of fly ash and garments waste fibers on the compressive and tensile strength of concrete specimens were evaluated through testing at 7 days, 28 days, and 90 days. Results showed that the compressive strength of specimens without fibers or fly ash increased over time, attributed to the formation of calcium-silicate-hydrate (C-S–H) and improved molecular bonding. However, the addition of garments fibers in proportions of 0.5% and 1% of concrete volume resulted in a decrease in compressive strength during the early curing stages. The inclusion of fly ash did not significantly affect strength in the shorter testing periods but positively influenced long-term strength at 90 days. Similarly, the tensile strength of the specimens increased with time, with higher values observed at 28 days and 90 days compared to 7 days. The presence of fibers contributed to additional tensile strength, with a more effect observed at a fiber ratio of 1%. While the addition of fly ash had limited influence on tensile strength in the shorter testing periods, it positively impacted the strength at 90 days. These findings suggest optimized combination of GWF and fly ash content can lead to improved mechanical proerties of concrete in the long run.