This study examined the influence of a pregnancy and postnatal multicomponent lifestyle intervention for women with gestational diabetes mellitus (GDM) and overweight/obesity from 6 weeks to 12 months postnatal. The primary outcome was weight at 12 months. Secondary outcomes included change in body mass index (BMI), waist circumference (WC) and fasting plasma glucose (FPG). The study involved 235 pregnant women with GDM and BMI ≥ 25 kg/m2 during pregnancy. Intervention components included an educational session, activity tracker (Fitbit), monthly phone calls, weekly motivational text messages, 12-week voucher for a commercial weight management organization and anthropometric, biochemical, and clinical measurements taken at 6 weeks, 6 months, and 12 months postnatal. The control group received routine local maternity care. A mean weight change of -2.0 (SD 7.1) kg was observed in the intervention group compared with -0.6 (SD 8.0) kg in the control group, difference -1.4 (95% CI -4.4, 1.5) kg from 6 weeks to 12 months postnatal, but this was not statistically significant (P = .34). Neither were significant differences obtained for any secondary outcomes: BMI -0.6 (-1.6, 0.5) kg/m2, WC -1.0 (-5.1, 3.2) cm and FPG 0.07 (-0.15, 0.29) mmol/L. This lifestyle intervention among women with overweight/obesity and GDM resulted in a statistically nonsignificant 1.4 kg greater weight loss compared with routine care at 12 months postnatal. Further research is needed to understand how the different components of this lifestyle intervention might be better applied to elicit more successful results.