Abstract Background Type 2 diabetes mellitus (T2DM) is a common chronic disease that disproportionally affects disadvantaged groups. People with a low socioeconomic position (SEP) have an increased risk of developing T2DM. Knowing the potential influence of early life on T2DM risk later in life, this study aims to examine longitudinal associations between childhood SEP and type 2 diabetes, independent of adult SEP. Methods Longitudinal data was used from 6,727 participants of The Maastricht Study without T2DM at baseline (participants with prediabetes were included in the study sample). T2DM status was determined by a standardized oral glucose test. Sample mean (SD) age was 58.7 (8.7) years and 55.2% was female. Childhood SEP was determined by the highest level of completed education for father and mother and a measurement for childhood income inadequacy. Adult SEP was determined by the highest completed level of education, equivalent household income and occupational position. Incident T2DM was self-reported yearly. Associations were studied annually over a 12-year period (median (IQR) 8.2 (4.9) years) with Cox regression analyses. Results A total of 247 participants (3.7%) without T2DM at baseline reported incident T2DM. Incident T2DM was more common in people with low childhood SEP versus high childhood SEP (4.5% vs. 2.7%, p = 0.008) and for people with low adult SEP versus high adult SEP (5.2% vs. 2.1%, p < 0.001). The impact of parental education on incident T2DM is mitigated by adult SEP. Childhood income inadequacy did predict T2DM independently of adult SEP, with a HR of 1.22 (1.01, 1.45). Conclusions Socioeconomic inequalities in childhood and adulthood are both predictive for incident T2DM. Experiencing income inadequacy in childhood increases the risk of developing T2DM in later life, independent of adulthood SEP. More attention is needed for deprivation in childhood and its impact on T2DM. Key messages • People who have experienced income inadequacy as a child have a higher risk of developing T2DM in later life, independent of their adult SEP. • More attention and research is needed for the impact of childhood poverty on developing T2DM in later life.