We systematically analysed the literature to assess whether diet and physical activity interventions delivered in high-income countries (HICs) could affect cardio-metabolic biomarkers among women with gestational diabetes mellitus (GDM) in pregnancy. Ten databases and two trial registries were searched from inception until 16/03/2026. Studies were included if they were randomised controlled trials in HICs that allocated women with GDM to diet and/or physical activity interventions, or a control group, and reported outcomes at 12 ± 3 months after the intervention. The review was preregistered in PROSPERO (CRD42024597324; CRD42024518659). We used random effects meta-analyses to pool data. We included 17 studies with 6,535 participants. There was evidence that interventions led to a significant reduction in weight (−0.89 kg (95% CI −1.74, −0.03); I2 = 57.7%), waist circumference (difference −0.79 cm (95% CI −1.45, −0.13); I2 = 0%), and systolic blood pressure (−1.60 mmHg (95% CI −3.16, −0.04), I2 = 72.1%). However, there was no evidence of a significant effect of these interventions on blood lipids, glycaemic outcomes, diastolic blood pressure, or type 2 diabetes incidence. The review highlights the need for high quality research on reducing the risk of long-term cardiometabolic disease among women with a history of GDM in HICs.