In 1990, Ridgeway was elected onto the first ATSIC Sydney Regional Council, a position in which he served two terms of office. He joined the Australian Democrats in 1991. In 1995, he became the executive director of the New South Wales Aboriginal Land Council. He continued in this position until 2000.
Ridgeway has served on a number of parliamentary and senate committees. In March 2001, he addressed the United Nations Special Session on Racism. He also attended the World Conference Against Racism in South Africa in the same year.
In April 2001, Ridgeway was elected deputy leader of the Democrats, after Natasha Stott Despoja toppled Meg Lees in a leadership spill.
Ridgeway continued as deputy leader until August 2002, when along with fellow senators John Cherry, Lyn Allison and Andrew Murray, he succeeded in forcing Stott Despoja to resign. It was widely tipped that he would succeed her as leader. However, the media did not portray the spill kindly, labelling them the 'Gang of Four'.
Ridgeway was expected to be appointed interim leader however, in a surprise decision, the party's National Executive installed Brian Greig as interim leader due to the role Ridgeway played in forcing the resignation of Stott Despoja. Ridgeway did not contest the subsequent leadership ballot, at which Andrew Bartlett defeated Brian Greig.
In the 2002 leadership ballot, Ridgeway lost his position as deputy leader, being replaced by Lyn Allison. He remained in the Senate, being particularly active on issues of indigenous affairs and reconciliation. He lost his seat in the 2004 election, along with Cherry, Lees (who had since left the party), and Greig. His term expired on 30 June 2005.