Ripper entered the Cabinet upon the formation of the Labor Government following the 2001 Western Australian election. He became Minister for Energy in 2001. Ripper had previously served as Deputy Premier, Treasurer, Minister for Government Enterprises and Minister Assisting the Minister for Public Sector Management in Gallop's cabinet.Prior to the 2008 election and the defeat of the Carpenter Labor government, he was the Deputy Premier and Treasurer, in the Carpenter government.
Ripper succeeded Alan Carpenter as WA leader of the ALP on 23 September 2008. In January 2011, Labor MP Ben Wyatt intended to challenge Ripper for the Labor leadership, but withdrew after finding minority support amongst caucus.