as a son of a diplomat, mertes spent most of his childhood and youth abroad (marseille, paris, moscow). in 1972 he graduated from high school where he had majored in classical languages. after a two-year military service from 1972 to 1974 he studied law at bonn and tübingen universities as well as at the london school of economics (with a focus on public international law, philosophy of law, philosophy of science). he completed his law degree at the beginning of 1981 and passed his bar exam in 1983.