In 1959, he became Secretary of the Christian Democratic Group of the European Parliament in Luxembourg; From 1965 to 1984 he was a member of the German Bundestag, representing Bergstraße. For many years he was a member of the Legal Affairs Committee, and he was its chairman from 1969 to 1980.
Lenz is especially committed to Franco-German friendship; he was a member of the Franco-German Parliamentary Group and its chairman from 1969 to 1983.
From 1984 to 1997, Lenz was Advocate General at the European Court of Justice. During this time, he drafted about 400 submissions to the Court of Justice. Lenz is also editor of a handbook and the author of numerous works concerning EC law.