Early-onset type 2 diabetes (defined as type 2 diabetes diagnosed in people aged <40 years) is increasingly prevalent with substantial health and socioeconomic implications. Unlike late-onset type 2 diabetes, early-onset type 2 diabetes is a high-risk and aggressive phenotype, with accelerated pancreatic β-cell decline and greater insulin resistance due to the rising rate of obesity. People with early-onset type 2 diabetes have higher rates of macrovascular and microvascular complications with increased health-care use and premature mortality (due to cardiovascular and non-cardiovascular complications) than do people with late-onset type 2 diabetes. Emerging evidence also suggests that people with early-onset type 2 diabetes face an increased risk of complications in reproductive health (eg, during periconception and postpartum periods), metabolic-associated steatotic liver disease, mental health (eg, diabetes distress, depression, anxiety, and psychotic disorders), and some cancers, creating additional challenges in managing multiple long-term conditions. In this Series paper, we highlight the consequences of early-onset type 2 diabetes and the key driver for these risks-long duration of exposure to hyperglycaemia, with its effects amplified by younger age at type 2 diabetes diagnosis and interactions with other cardiometabolic risk factors. Recognising these adverse risks associated with early-onset type 2 diabetes is crucial for guiding the development and implementation of a more focused and integrated life-course approach to mitigate its long-term effect on individuals, communities, and health-care systems globally. However, substantial research gaps remain that must be addressed, particularly in diverse populations.