Mercury emissions have increased due to modern human activities, posing a serious marine pollution issue due to its long residence time in the ocean, high biomagnification potential, and toxicity. Seabirds are one of the groups which are highly exposed to mercury contamination. However, it has been controversial issues about which seabird species in which regions are exposed to high levels of mercury contamination. In this study, we addressed this question via a global meta-analysis using data of blood total mercury concentration in seabirds (mean values per species/year/location) collected through literature review and our own sampling. The blood mercury levels in adults, especially during the breeding period, could reflect mercury intake around the sampling site within 1–2 months prior to sampling. We obtained 473 data points from 109 species across 13 families worldwide. Health risk assessments of blood mercury, based on a previous study, showed that 84 out of 109 species were at low risk levels, while 25 species, including some albatrosses, procellariforms, and terns, exhibited moderate to severe mercury levels. We analyzed ecological factors—including body mass, main diet, foraging type (diver or surface feeder), and typical foraging range—as well as spatial factors—including chlorophyll-a concentration, latitude around the sampling locations, and ocean region—that may influence blood mercury levels, using a linear model. Blood mercury levels were higher in species with larger body mass, those that feed on higher trophic-level prey such as fish, those that feed on mesopelagic prey with high mercury content. Birds from areas with lower chlorophyll-a concentrations had higher blood mercury levels, suggesting that in low-productivity regions, the smaller phytoplankton and longer food chains contribute to greater biomagnification of mercury in top predators. We also found significant effects of ocean region on blood mercury; seabirds in the North Pacific and North Atlantic had higher mercury levels, while those in polar regions did not consistently exhibit elevated contamination. Although previous studies have suggested high mercury pollution in polar regions, our findings did not support this trend. In conclusion, while blood mercury levels of seabirds were partly explained by diet, significant spatial variability was observed. Our study would be helpful not only in clarifying the mercury contamination mechanism in the marine ecosystem but also in applying seabirds as bioindicators to assess the global spatial distribution of mercury pollution.