The effects of an alkali treatment with NaOH (5%, 10%, and 15%) was studied relative to the tensile and flexural mechanical properties of Colombian coffee silverskin fiber (CCSF)/polyester biocomposites. The CCSF was treated with NaOH for 30 min and then dried at 50 °C for 5 h. Laminates with a 35.1% volume fraction of fiber were prepared using a hand lay up manufacturing technique. Scanning electron microscopy was used to study the fiber morphology. The results revealed that the specific maximum tensile and flexural strengths increased by 36.3% and 25.1%, respectively, the specific tensile and flexural moduli increased by 31.6% and 147.0%, respectively, and the tensile toughness was 67.9% higher compared with the untreated biocomposite. The flexural toughness decreased by 62.0%. The results suggested that mercerization is an effective method to treat CCSF/polyester biocomposites.