Ridgway was elected state president of the South Australian Rural Youth Movement in 1982–83. In 1984 he won a six-month youth study tour to the UK. This experience heightened his interest in politics, especially in primary industries and regional development.
Ridgway began working on the family farm and at 19 he took on management of the family's horticultural business. After purchasing the business in 1997 with his wife Meredith, they expanded the operation to become the largest producer of gladioli corms in Australia and New Zealand.
Ridgway was elected from fourth position on the Liberal ticket at the 2002 election and from first position on the Liberal ticket at the 2010 election.
Ridgway was appointed a Shadow Parliamentary Secretary in 2005, and after the 2006 election was appointed Shadow Minister for Environment and Conservation, the River Murray and Urban Development & Planning, under the leadership of Iain Evans. A leadership change in April 2007, in which Martin Hamilton-Smith assumed Liberal Party leadership, prompted a re-shuffle of the Party's Shadow Cabinet, at which point Ridgway was made Shadow Minister for Police and Mineral Resources Development, whilst retaining his Shadow Urban Development & Planning post. David Ridgway also assisted the Leader of the Opposition with the Multicultural Affairs portfolio. In February 2013, Ridgway was further promoted to the crucial portfolios of Agriculture, Food and Fisheries, Forests and Tourism. Between 2007 and 2018 Ridgway served as the Leader of the Opposition in the Legislative Council.
Following the 2018 state election Ridgway was appointed as Minister for Trade, Tourism and Investment.[3]
Ridgway was relieved[4] of his Tourism ministerial responsibilities in the immediate aftermath of the devastation of Kangaroo Island and other South Australian tourist areas during the 2019–20 bushfires when the Premier, Steven Marshall assumed that ministerial role on 11 January 2020.[4] On 13 January 2020, by proclamation of the Governor, Ridgway was relieved of the Tourism portfolio in his ministerial position.[5] In July 2020, Ridgway resigned from cabinet, indicating he was unwilling to serve in a reshuffled ministry following the resignation of Stephan Knoll and Tim Whetstone over an expenses scandal.[6]
Ridgway resigned from the Legislative Council in June 2021 in order to be appointed as South Australia's Agent-General in London.[7]
