Adequate glycemic management is one of the main goals in treating type 1 diabetes mellitus (T1DM) and preventing the early onset of diabetic complications. Improperly controlled diabetes mellitus (DM) will result in oxidative stress (OS) and lead to further related health issues. Therefore, the aim of this study was to evaluate the body’s ability to defend against OS depending on the duration of T1DM, metabolic management, antioxidant intake and modern glycemic monitoring systems (GMS). The study included 103 adolescents with T1DM aged 10–17 years. The control group consisted of 65 healthy peers. The patients’ blood was assayed for antioxidant enzymes, minerals and toxic elements. In addition, their dietary intake of antioxidant components was assessed. The T1DM group had higher total oxidant status, oxidative stress index and Cu/Zn ratio values, higher concentrations of malondialdehyde and lower total antioxidant status (TAS) and chromium, zinc, superoxide dismutase and catalase levels than their healthy peers. The comparison between GMS types revealed favorable changes in OS parameters for the flash and continuous systems. Furthermore, an effect of vitamin A and C dietary intake on serum TAS concentrations was detected. More than 82% of the patients with high TAS fulfilled the estimated average requirement norm for vitamin A, and more than 60% fulfilled the vitamin C requirement. In youths with T1DM, it is advisable to observe the antioxidant activity of the body to prevent the accelerated development of diabetic complications.