Introduction: Among patients regularly undergoing hemodialysis, hypocarnitinaemia often develops as a consequence of inadequate dietary intake, reduced synthesis in the body, and considerable losses during hemodialysis. Objectives: To evaluate the effects of L-carnitine supplementation on patients with end-stage kidney disease (ESKD) who underwent hemodialysis. Methods: Thirty-one patients with ESKD, comprising 18 men and 13 women, with a median age of 72 (range 58–89) years, who underwent regular hemodialysis received treatment with L-carnitine for 1 year. The total and free carnitine, acylcarnitine, and amino acids (AA) levels before and after L-carnitine treatment were analyzed, and the blood biochemistry results and clinical profiles of the subjects were compared before and after treatment. Results: The median (interquartile range [IQR]) serum total and free carnitine and acylcarnitine levels significantly increased from 34.5 (28.2–44.3), 20.9 (15.8–27.6), and 14.1 (11.2–17.6) µmol/L, respectively to 407.4 (371.6–493.5), 270.2 (228.3–316.0), and 155.0 (136.1–168.5) µmol/L, respectively, after treatment (all p < 0.001). The median (IQR) blood valine, tyrosine, phenylalanine, and citrulline levels increased from 0.94 (0.80–1.09), 0.45 (0.39–0.55), 0.61 (0.56–0.79), and 1.04 (0.79–1.26) mg/dL, respectively to 1.24 (1.13–1.54), 0.76 (0.62–0.85), 0.90 (0.70–1.04), and 1.22 (0.92–1.39) mg/dL, respectively, following L-carnitine treatment ( p < 0.001, p < 0.001, p = 0.002, and p = 0.030, respectively); however, the median (IQR) blood arginine level decreased from 0.20 (0.13–0.24) to 0.09 (0.06–0.14) mg/dL after treatment ( p < 0.001). The median (IQR) percentage fractional shortening (41.5 vs. 41.9%; p = 0.012) and left ventricular ejection fraction (65.2 vs. 67.3%; p = 0.036) increased significantly following treatment. Conclusions: L-Carnitine increased the blood acylcarnitine levels, enhanced fatty acid metabolism, and affected AAs metabolism; this may be beneficial for energy production within the cardiac and skeletal muscles.