Evidence suggests that maternal prepregnancy body mass index (BMI) is associated with offspring cardiometabolic risk factors. This study was aimed at assessing the association of maternal prepregnancy BMI with offspring cardiometabolic risk factors in adolescence and adulthood. We also evaluated whether offspring BMI was a mediator in this association. The study included mother–offspring pairs from three Pelotas birth cohorts. Offspring cardiometabolic risk factors were collected in the last follow-up of each cohort [mean age (in years) 30.2, 22.6, 10.9]. Blood pressure was measured using an automatic device, cholesterol by using an enzymatic colorimetric method, and glucose from fingertip blood, using a portable glucose meter. In a pooled analysis of the cohorts, multiple linear regression was used to control for confounding. Mediation analysis was conducted using G-computation formula. In the adjusted model, mean systolic blood pressure of offspring from overweight and obese mothers was on average 1.25 (95% CI: 0.45; 2.05) and 2.13 (95% CI: 0.66; 3.59) mmHg higher than that of offspring from normal-weight mothers; for diastolic blood pressure, the means were 0.80 (95% CI: 0.26; 1.34) and 2.60 (95% CI: 1.62; 3.59) mmHg higher, respectively. Non-HDL cholesterol was positively associated with maternal BMI, whereas blood glucose was not associated. Mediation analyses showed that offspring BMI explained completely the association of maternal prepregnancy BMI with offspring systolic and diastolic blood pressure, and non-HDL cholesterol. Our findings suggest that maternal prepregnancy BMI is positively associated with offspring blood pressure, and blood lipids, and this association is explained by offspring BMI.