In 1974 at the age of 18 he became an organiser for the Progressive Party, one of the two opposition parties represented in parliament at the time. After this, he qualified as an attorney at the University of the Witwatersrand, where he was President of the Law Students' Council and Vice-President of the Students' Representative Council, and became a lecturer in the Law Department in 1986. He has accredited Harry Schwarz and Helen Suzman as his biggest inspirations. In the same year he was elected to the Johannesburg City Council for Yeoville. When the results for the election were released, it was announced that the NP candidate Sam Bloomberg had won. However Harry Schwarz, his political mentor, uncovered that this was untrue, and Leon was declared the winner. He became leader of the opposition in the city council.