Small and nutritionally at-risk infants under six months, defined as those with wasting, underweight, or other forms of growth failure, are at high-risk of mortality and morbidity. The World Health Organisation 2013 guidelines on severe acute malnutrition highlight the need to effectively manage this vulnerable group, but programmatic challenges are widely reported. This review aims to inform future management strategies for small and nutritionally at-risk infants under six months in low- and middle-income countries (LMICs) by synthesising evidence on existing breastfeeding support packages for all infants under six months. We searched PubMed, CINAHL, Cochrane Library, EMBASE, and Global Health databases from inception to 18 July 2018. Intervention of interest were breastfeeding support packages. Studies reporting breastfeeding practices and/or caregivers'/healthcare staffs' knowledge/skills/practices for infants under six months from LMICs were included. Study quality was assessed using NICE quality appraisal checklist for intervention studies. A narrative data synthesis using the Synthesis Without Meta-analysis (SWiM) reporting guideline was conducted and key features of successful programmes identified. Of 15,256 studies initially identified, 41 were eligible for inclusion. They were geographically diverse, representing 22 LMICs. Interventions were mainly targeted at mother-infant pairs and only 7% (n = 3) studies included at-risk infants. Studies were rated to be of good or adequate quality. Twenty studies focused on hospital-based interventions, another 20 on community-based and one study compared both. Among all interventions, breastfeeding counselling (n = 6) and education (n = 6) support packages showed the most positive effect on breastfeeding practices followed by breastfeeding training (n = 4), promotion (n = 4) and peer support (n = 3). Breastfeeding education support (n = 3) also improved caregivers' knowledge/skills/practices. Identified breastfeeding support packages can serve as "primary prevention" interventions for all infants under six months in LMICs. For at-risk infants, these packages need to be adapted and formally tested in future studies. Future work should also examine impacts of breastfeeding support on anthropometry and morbidity outcomes. The review protocol was registered in the International Prospective Register of Systematic Reviews (PROSPERO 2018 CRD42018102795).