Background: Two marine fungi, a Trichoderma sp. and a Coniochaeta sp., which can grow on d-galacturonic acid and pectin, were selected as hosts to engineer for mucic acid production, assessing the suitability of marine fungi for production of platform chemicals. The pathway for biotechnologcial production of mucic (galactaric) acid from d-galacturonic acid is simple and requires minimal modification of the genome, optimally one deletion and one insertion. d-Galacturonic acid, the main component of pectin, is a potential substrate for bioconversion, since pectin-rich waste is abundant. Results: Trichoderma sp. LF328 and Coniochaeta sp. MF729 were engineered using CRISPR-Cas9 to oxidize d-galacturonic acid to mucic acid, disrupting the endogenous pathway for d-galacturonic acid catabolism when inserting a gene encoding bacterial uronate dehydrogenase. The uronate dehydrogenase was expressed under control of a synthetic expression system, which fucntioned in both marine strains. The marine Trichoderma transformants produced 25 g L −1 mucic acid from d-galacturonic acid in equimolar amounts: the yield was 1.0 to 1.1 g mucic acid [g d-galacturonic acid utilized] −1. d-Xylose and lactose were the preferred co-substrates. The engineered marine Trichoderma sp. was more productive than the best Trichoderma reesei strain (D-161646) described in the literature to date, that had been engineered to produce mucic acid. With marine Coniochaeta transformants, d-glucose was the preferred cosubstrate, but the highest yield was 0.82 g g −1 : a portion of d-galacturonic acid was still metabolized. Coniochaeta sp. transformants produced adequate pectinases to produce mucic acid from pectin, but Trichoderma sp. transformants did not. Conclusions: Both marine species were successfully engineered using CRISPR-Cas9 and the synthetic expression system was functional in both species. Although Coniochaeta sp. transformants produced mucic acid directly from pectin, the metabolism of d-galacturonic acid was not completely disrupted and mucic acid amounts were low. The d-galacturonic pathway was completely disrupted in the transformants of the marine Trichoderma sp., which produced more mucic acid than a previously constructed T. reesei mucic acid producing strain, when grown under similar conditions. This demonstrated that marine fungi may be useful as production organisms, not only for native enzymes or bioactive compounds, but also for other compounds.