Although cell therapy improves cardiac function after myocardial infarction, highly variable results and limited understanding of the underlying mechanisms preclude its clinical translation. Because many heart failure patients are diabetic, we examined how diabetic conditions affect the characteristics of cardiac mesenchymal cells (CMC) and their ability to promote myocardial repair in mice. To examine how diabetes affects CMC function, we isolated CMCs from non-diabetic C57BL/6J (CMC^WT) or diabetic B6.BKS(D)-Leprdb/J (CMC^db/db) mice. When CMCs were grown in 17.5 mM glucose, CMC^db/db cells showed > twofold higher glycolytic activity and a threefold higher expression of Pfkfb3 compared with CMC^WT cells; however, culture of CMC^db/db cells in 5.5 mM glucose led to metabolic remodeling characterized by normalization of metabolism, a higher NAD⁺/NADH ratio, and a sixfold upregulation of Sirt1. These changes were associated with altered extracellular vesicle miRNA content as well as proliferation and cytotoxicity parameters comparable to CMC^WT cells. To test whether this metabolic improvement of CMC^db/db cells renders them suitable for cell therapy, we cultured CMC^WT or CMC^db/db cells in 5.5 mM glucose and then injected them into infarcted hearts of non-diabetic mice (CMC^WT, n = 17; CMC^db/db, n = 13; Veh, n = 14). Hemodynamic measurements performed 35 days after transplantation showed that, despite normalization of their properties in vitro, and unlike CMC^WT cells, CMC^db/db cells did not improve load-dependent and -independent parameters of left ventricular function. These results suggest that diabetes adversely affects the reparative capacity of CMCs and that modulating CMC characteristics via culture in lower glucose does not render them efficacious for cell therapy.