Cardiolipin (CL) is the signature phospholipid of the inner mitochondrial membrane, where it stabilizes electron transport chain protein complexes1. The final step in CL biosynthesis relates to its remodelling: the exchange of nascent acyl chains with longer, unsaturated chains1. However, the enzyme responsible for cleaving nascent CL (nCL) has remained elusive. Here, we describe ABHD18 as a candidate deacylase in the CL biosynthesis pathway. Accordingly, ABHD18 converts CL into monolysocardiolipin (MLCL) in vitro, and its inactivation in cells and mice results in a shift to nCL in serum and tissues. Notably, ABHD18 deactivation rescues the mitochondrial defects in cells and the morbidity and mortality in mice associated with Barth syndrome. This rare genetic disease is characterized by the build-up of MLCL resulting from inactivating mutations in TAFAZZIN (TAZ), which encodes the final enzyme in the CL-remodelling cascade1. We also identified a selective, covalent, small-molecule inhibitor of ABHD18 that rescues TAZ mutant phenotypes in fibroblasts from human patients and in fish embryos. This study highlights a striking example of genetic suppression of a monogenic disease revealing a canonical enzyme in the CL biosynthesis pathway.