Carboxylic acid reductases (CARs) catalyze the reduction of a broad range of carboxylic acids to aldehydes using the co-factors ATP and NADPH, and have become attractive biocatalysts for organic synthesis. Here we exploit our mechanistic understanding of CARs to expand their reaction scope, generating biocatalysts for amide bond formation from carboxylic acid and amine. After reaction engineering, CARs were found to have amidation activity for various acids and amines. Optimization of reaction conditions with respect to pH and temperature allowed for the synthesis of the anticonvulsant ilepcimide with up to 96% conversion. Mechanistic studies using site-directed mutagenesis suggest that following initial enzymatic adenylation of substrates, amidation of the carboxylic acid proceeds via direct reaction of the acyl adenylate with amine nucleophiles.