Mercury (Hg) bioaccumulates in the aquatic food chain in the form of methylmercury, a compound well known for its neurotoxicity. We analyzed total mercury (THg) in hair collected from 209 harbour seals captured at 10 sites in British Columbia (Canada) and Washington State (USA) between 2003 and 2010. In addition, laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) allowed for a highly refined analysis of THg accumulation over time by examining nine whiskers taken from 4- to 6-week-old pups. We estimate that THg concentrations in pups increased sharply at a point corresponding to mid- to late gestation of their time in utero (4.7 ± 0.8 and 6.6 ± 1.3 µg/g dry weight (dw), respectively), and then again at the onset of nursing (8.1 ± 1.3 µg/g dw). These abrupt changes highlight the importance of both pre- and post-natal THg transfer from the mother to the growing fetus and the newborn pup. While THg levels varied among sites, hair analyses from seals collected at the same site demonstrated the influence of age in THg accumulation with pups (5.3 ± 0.3 µg/g) and juveniles (4.5 ± 0.5 µg/g) having lower levels than those in adults (8.3 ± 0.8 µg/g). Our results revealed that 33 % of the pups sampled (n = 167) had THg levels that surpassed a mammalian hair threshold for neurochemical alterations. This study suggests that Hg could represent a health concern to marine wildlife, especially as atmospheric emissions of this toxic element from human activities in the Pacific Rim and worldwide continue.