We used free-ranging northern elephant seals as a model species to investigate maternal transfer of mercury (Hg) to pups in relation to maternal traits, including maternal blood mercury concentrations, and to quantify the links between pup lanugo mercury concentrations and maternal mesopelagic foraging behavior during gestation. We satellite tracked known-age adult females at sea during their seven-month-long foraging trip to determine maternal foraging strategies (geographic locations and diving depths) and quantified mercury concentrations in maternal blood and their pups’ blood and lanugo (which is defined as pup hair grown in utero) early in lactation when seals were hauled out. We then tested how the maternal transfer of mercury to pups is influenced by additional maternal traits, such as maternal age, as well as for intra-year variability by including additional lanugo samples from more than 300 pups of known adult females that we did not handle or track at sea.