The World Health Organization recommends exclusive breastfeeding up to the age of six months and continuation of partial breastfeeding up to the age of two years, in addition to nutritionally adequate and safe food. In Mauritania, despite some progress, most mothers do not comply with these recommendations. The aim of this study, conducted in Nouakchott, was to evaluate breastfeeding and feeding practices, and measure factors associated with achievement of the optimal duration of exclusive breastfeeding. The methodology combined quantitative and qualitative approaches. A descriptive cross-sectional study was conducted by questionnaires among 330 mothers from different departments of the capital. Twenty semi-structured interviews were then conducted with Mauritanian grandmothers in order to understand their roles and perceptions about infant feeding. Before the age of 6 months, the exclusive breastfeeding rate was 18.4%, the predominant breastfeeding rate was 44.3% and the partial breastfeeding with milk rate was 28.1%. In addition, 9.2% of infants received supplementary feeding. We found that 50.5% of mothers were aware of the optimal duration of exclusive breastfeeding, but only 14.2% complied with this recommendation. The factors significantly associated with compliance with the optimal duration of exclusive breastfeeding were maternal age over 35 years and multiparity. Interviews revealed that grandmothers knew about some of the nutritional recommendations, but denied their relevance based on their experience. Their advice contradicted certain medical recommendations. Our study revealed inadequacies concerning the mother’s knowledge and more frequently their practices in terms of infant feeding. The gap between knowledge and practice can be essentially explained by the relative importance attributed to recommendations by the mothers, as well as the confrontation between medical recommendations and grandmothers’ traditional knowledge.