Mercury in seafood is a neurotoxicant that threatens human health. Dynamic rates of mercury emission, re-emission, and atmospheric deposition warrant studies into mercury concentrations in fish because many are consumed by humans and can serve as sentinels of mercury levels in the environment. We modeled trends in total mercury content in an apex marine fish predator, Atlantic blue marlin Makaira nigricans, whose muscle tissues were opportunistically sampled from North Carolina (USA) sportfishing tournaments over a discontinuous time period: between 1975 and 77 and 1998–2021 (n = 148). The model-estimated influence of marlin weight on total mercury concentration was constant across years (shared slope) allowing for comparisons of weight-corrected mercury concentrations among years. Weight-corrected total mercury concentrations revealed an inter-decadal decline of approximately 45 % between the 1970s and late 1990s and then variable but relatively stable concentrations through 2021. The mean (SD) wet weight concentration of total mercury was 9.47 (4.11) from 1975 to 77 and 4.17 (2.61) from 2020 to 2021. Methylmercury and selenium were measured on a subset of fish to address questions related to human health and consumption. Methylmercury levels (mean = 0.72 μg/g) were much lower than total mercury (mean = 4.69 μg/g) indicating that total mercury is not a good proxy for methylmercury in Atlantic blue marlin. Selenium, examined as a Se:Hg molar ratio and as a selenium health benefit value (HBVSe), showed high protective value against mercury toxicity. Long-term trends in the concentration of mercury in blue marlin should continue to be monitored to determine whether policies to mitigate anthropogenic contributions to global mercury are achieving their intended goals and to provide information to inform safe human consumption. Display omitted •Long-term studies of offshore marine apex predators are valuable for indexing ambient heavy metal concentrations.•Atlantic blue marlin muscle collected from 1975 to 2021 was analyzed for total mercury, methylmercury, and selenium.•Total mercury concentrations declined from the 1970s to late 1990s, but remained approximately constant during this century.•Methylmercury is a small percentage of total mercury in Atlantic blue marlin, which is not typically observed in fish.•High selenium concentrations in Atlantic blue marlin tissue may substantially offset mercury toxicity in human consumers.