Background/Objectives: To evaluate pancreatic size and echogenicity using ultrasonography in newly diagnosed pediatric Type 1 Diabetes Mellitus patients within five days of diagnosis, and compare early childhood (<7 years) and adolescent (≥13 years) endotypes with clinical and laboratory findings. Methods: This prospective, cross-sectional, case-control study included 69 pediatric patients with newly diagnosed type 1 diabetes mellitus, aged 1-18 years, and 78 age- and sex-matched healthy controls. Patients with chronic conditions (e.g., pancreatitis or cystic fibrosis), other forms of diabetes, or medications affecting glucose metabolism were excluded. Ultrasonography was performed within five days of diagnosis, after metabolic stabilization, to assess pancreatic dimensions and echogenicity. Laboratory analyses included measurements of C-peptide, HbA1c, and autoantibodies (anti-GAD, islet cell antibody, and insulin antibody). Results: Pancreatic dimensions were significantly smaller in type 1 diabetes mellitus patients (p < 0.001), with greater reductions in adolescents (head: 21%, body: 26.7%) vs. young children (head: 14.4%, body: 15.5%). Isoechoic pancreases were more common in young patients (80% vs. 40.9%; p = 0.033). C-peptide and HgbA1c were higher in adolescents (p < 0.05), with no echogenicity-autoantibody association. Conclusions: This first early-post-diagnosis ultrasonography study reveals age-specific pancreatic atrophy and echogenicity changes in children, more severe in adolescents, reflecting type 1 diabetes mellitus endotypes. Ultrasonography offers a practical noninvasive tool for early detection and endotype stratification, informing personalized diabetes care.