S ‐adenosyl‐l‐methionine (SAM)‐dependent methyltransfer ‐ases (MT) are an important class of enzymes, catalysing methylation of a vast array of small metabolites and biomacromolecules. Recently, rare carboxymethylation pathways have been discovered, including carboxymethyltransferase enzymes that utilise a carboxy‐SAM (cxSAM) cofactor, generated from SAM by a cxSAM synthase (CmoA). In this study we show how MT enzymes can utilise cxSAM, catalysing carboxymethylation of tetrahydroisoquinoline (THIQ) and catechol substrates. Site‐directed mutagenesis was used to create orthogonal MT possessing improved catalytic activity and selectivity for cxSAM, with subsequent coupling to CmoA resulting in more efficient and selective carboxymethylation. An enzymatic approach was also developed to generate a previously undescribed co‐factor, carboxy‐ S ‐adenosyl‐l‐ethionine (cxSAE), enabling the stereoselective transfer of a chiral 1‐carboxyethyl group to the substrate. These findings provide a platform by which common methylation pathways can be engineered to deliver carboxymethylated products, providing new properties and functionality.