Dietary analysis predicts that marginal Zn deficiency is common in women of reproductive age. The lack of reliable biomarkers limits the capacity to assess Zn status and consequently understand effects of maternal Zn deficiency. We determined effects of marginal maternal Zn deficiency on mammary gland function, milk secretion, and milk composition in mice. Mice (n = 12/diet) were fed marginal (ZD; 15 mg Zn/kg diet) or adequate (ZA; 30 mg Zn/kg diet) Zn diets for 30 d prior to conception through mid-lactation. Mice fed the ZD had a higher plasma Zn concentration (~20%; P < 0.05) but lower milk Zn concentration (~15%; P < 0.05) compared with mice fed the ZA. ZnT2 abundance was higher (P < 0.05) in mice fed the ZD compared with mice fed the ZA; no effect on ZnT4 abundance was detected. The Zn concentration of mammary gland mitochondria tended to be ~40% greater in mice fed ZD (P = 0.07); this was associated with apoptosis and lower milk secretion (~80%; P < 0.01). Total milk protein was ~25% higher (P < 0.05), although the abundance of the major milk proteins (caseins and whey acidic protein) was lower (P < 0.05) in mice fed the ZD. Proteomic analysis of milk proteins revealed an increase (P < 0.05) in four proteins in mice fed the ZD. These findings illustrate that marginal maternal Zn deficiency compromises mammary gland function and milk secretion and alters milk composition. This suggests that lactating women who consume inadequate Zn may not produce and/or secrete an adequate amount of high quality milk to provide optimal nutrition to their developing infant.