Fermented Miang (Camellia sinensis var. assamica) serves as a valuable source of bioactive polyphenols and probiotic-associated components. This study characterized the catechin composition of fermented Miang extracts and evaluated their antioxidant capacity and suitability for cosmetic formulations. High-performance liquid chromatography (HPLC) analysis showed that epigallocatechin gallate (EGCG) was the predominant catechin (7.00 ± 0.93 mg/g dry weight), followed by catechin (C), epicatechin (EC), epicatechin gallate (ECG), and epigallocatechin (EGC). The extracts remained physically and chemically stable for at least three months under various storage conditions, with the dried extract form offering advantages for handling and formulation. Fermentation duration significantly influenced phenolic accumulation and antioxidant activity, with four-month fermentation showing the highest activity. Prototype cleansing formulations, including transparent/opaque soap bars, liquid soap, and shampoo containing fermented Miang extract, exhibited acceptable physicochemical characteristics and retained antioxidant function. These findings highlight fermented Miang as a promising natural ingredient for antioxidant and probiotic-inspired cosmetic applications.