The complexity of trade networks is a major challenge to controlling wildlife trafficking and illegal, unreported, and unregulated (IUU) fishing. These networks may not be modern inventions, but have developed over centuries, from integrated global markets that preceded modern regulatory policies. To understand these linkages, we curated 150 years of tortoiseshell transactions and derived biologically informed harvest models to estimate the trade in critically endangered hawksbill sea turtles (Eretmochelys imbricata). We find that trade networks concentrated in Southeast Asia harvested 9 million turtles, over six times previous estimates. These networks spread from within the Pacific, to the Indian and Atlantic basins, and became markedly more complex after 1950. Our results further indicate that the magnitude and extent of the coastally restricted hawksbill exploitation parallel current patterns of IUU fishing. Policies to combat these interlinked illegal practices should assimilate the important role of small-scale, coastal fisheries in these increasingly complex global networks.