Therapeutic monoclonal antibodies (mAbs) are used to manage a wide range of cancers and autoimmune disorders. However, mAb-based treatments are not always successful, highlighting the need for a better understanding of the factors influencing mAb efficacy. Increased levels of oxidative stress associated with several diseases are counteracted by the activities of various oxidoreductase enzymes, such as thioredoxin (Trx), which also reduces allosteric disulfide bonds in proteins, including mAbs. Here, using an array of in vitro assays, we explored the functional effects of Trx-mediated reduction on the mechanisms of action of six therapeutic mAbs. We found that Trx reduces the inter-chain disulfide bonds of the mAbs, after which they remain intact but have altered function. In general, this reduction increased antigen-binding capacity, resulting in, for example, enhanced tumour necrosis factor (TNF) neutralization by two anti-TNF mAbs. Conversely, Trx reduction decreased the anti-proliferative activity of an anti–tyrosine kinase-type cell-surface receptor HER2 (HER2) mAb. In all the mAbs, Fc receptor binding was abrogated by Trx activity, with significant loss in both complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) activity of the mAbs tested. We also confirmed that without alkylation, Trx-reduced inter-chain disulfide bonds reoxidize, and ADCC activity is restored. In summary, Trx-mediated reduction has a substantial impact on the functional effects of an mAb, including variable effects on antigen binding and Fc function, with the potential to significantly impact mAb efficacy in vivo.