This study explores the antioxidant activity of green tea (Camellia sinensis, GT) and peppermint (Mentha × piperita, PM) infusions, individually and in combination, focusing on how preparation methods affect their efficacy. Antiradical and intracellular antioxidant activity was assessed using DPPH, ABTS, and DCF assays, alongside interaction analysis via combination index (CI) and dose reduction index (DRI). HPLC analysis determined the polyphenolic profiles of GT and PM. GT showed the strongest antioxidant activity, with the lowest IC₅₀ values (4.81 µg/mL in DPPH, 2.70 µg/mL in ABTS, 3.71 µg/mL in DCF), indicating potent radical-scavenging potential. PM exhibited moderate antiradical capacity but similar intracellular activity (IC₅₀ = 3.80 µg/mL). Co-maceration followed by lyophilization of GT:PM extracts led to nearly additive interactions (CI~1.0) and allowed significant dose reduction (DRI up to 4.44). In contrast, post-mixed extracts showed assay-dependent effects, including antagonism in intracellular ROS inhibition (CI = 1.83). Equimolar mixtures of model polyphenols: EGCG, quercetin, and rosmarinic acid demonstrated enhanced effects, with the strongest synergy in ternary mixtures (CI = 0.67–0.86), potentially achievable in GT:PM combinations. These findings highlight that extract preparation critically influences antioxidant efficacy, supporting co-maceration as a promising strategy for developing effective functional formulations based on plant extract combinations.