Family-based eHealth interventions to reduce cardiovascular disease risk have potential as a primary prevention strategy to improve the health of parents and their children. This systematic review evaluated the effectiveness of such interventions in modifying parent and child/adolescent risk factors such as body mass index, physical activity, dietary intakes and alcohol use. Five electronic databases were searched up to April 2020. Of 2193 articles identified, seven randomised controlled trials met inclusion criteria and were reviewed. Data were extracted regarding study setting, design, methods, eHealth technology used, intervention and control group components, retention rates, outcome measures, incentives and limitations. Risk of bias and quality assessment were carried out using Cochrane methods. A qualitative narrative data synthesis of the studies was conducted. Our review found that three studies showed an improvement in alcohol use among parents and adolescents as a result of the eHealth intervention. Among children/adolescents, two studies showed an improvement in dietary intake, one study showed an improvement in physical activity, and one study showed an improvement in body mass index as a result of the eHealth intervention. Interventions appeared more likely to be effective if they were theory-based, had longer follow-up periods, were incentivised and included regular interaction. Our findings suggest that, despite a paucity of high-quality trials, there is some evidence that family-based eHealth interventions have potential to reduce cardiovascular disease risk. However, more sufficiently powered, higher-quality trials with theory driven, clearly described interventions and unambiguous outcomes are needed.