Long-chain fatty acid oxidation disorders (LC-FAODs) cause energy deficits in heart and skeletal muscle that are only partially corrected by current medium-chain lipid therapies such as triheptanoin. We find that heart and muscle lack medium-chain acyl-CoA synthetases, limiting the capacity for β-oxidation of medium-chain fatty acids. Instead, heart and muscle mitochondria robustly respire on medium-chain acylcarnitines. The mitochondrial matrix enzyme carnitine acetyltransferase (CrAT) efficiently converts orally delivered octanoylcarnitine (C8-carnitine) to octanoyl-CoA for energy generation. C8-carnitine exhibits twice the oral bioavailability of triheptanoin and distributes to muscle and heart. A single oral dose significantly enhances grip strength and treadmill endurance while attenuating lactic acidosis in 2 mouse models of LC-FAODs. Thus, medium-chain acylcarnitines overcome a previously unrecognized metabolic bottleneck in LC-FAOD muscle and may represent an alternative to triglyceride-based therapies for bioenergetic disorders.