1990s

As the Liberal Party state president from 1992–95, Chapman attempted to win Liberal preselection for the federal division of Barker in 1998. Her husband, David, died in 2001 and she moved from Wayville to Tusmore with her two children. She again tried to win preselection, this time for the safest Liberal seat in the metropolitan area, Bragg, located in Adelaide's wealthy eastern suburbs.

2000s

When sitting member Graham Ingerson resigned, Chapman contested preselection against Liberal minister Michael Armitage, who was seeking to move from his marginal seat of Adelaide. Chapman easily gained preselection and retained Bragg with a slight 0.4-point two-party swing at the 2002 state election when the Liberals lost government. However, she won 61.9 percent of the primary vote, easily enough to retain the seat outright.

Chapman immediately joined the front bench, assuming the shadow portfolios of Education and Children's Services. She was soon touted by some quarters, within her party and in the media, as a future Liberal leader. In other quarters, however, Chapman was seen as a continuation of the factional battles that have long plagued the SA Liberals.

The Liberals were cut down to only 15 seats at the 2006 election landslide. Chapman herself suffered a substantial 6.8-point two-party swing, but still retained Bragg with a comfortable majority of 12.6 percent, leaving Bragg as the only safe Liberal seat in Adelaide and one of only four safe Liberal seats statewide. Chapman was elected as deputy Liberal leader, and hence Deputy Leader of the Opposition, in an unexpected joint ticket with factional rival Iain Evans. Strong backing was received from federal Sturt MP Christopher Pyne, a longtime factional ally of Chapman, as well as another prominent boss of the SA Liberals' moderate faction, former Premier Dean Brown.^[2] She retained the deputy's post when Martin Hamilton-Smith ousted Evans as leader in 2007.

Hamilton-Smith called a leadership and deputy leadership spill for 4 July 2009. Chapman ran against Hamilton-Smith for the leadership, but received only 10 votes, against Hamilton-Smith's 11, with Evans abstaining. Conservative Isobel Redmond was elected to the deputy leadership to replace Chapman.^[3] Hamilton-Smith called another leadership spill to take place on 8 July 2009, in an attempt to gain a more decisive mandate, but two days prior to the spill, he announced he would not run. Chapman again ran for the leadership but received only 9 votes, against Redmond's 13. Steven Griffiths was elected deputy leader 8 votes to 6 for Mitch Williams.^[4]

2010s

Despite having attempted to previously oust Hamilton-Smith as leader and having attempted to later defeat Redmond in a leadership ballot, Chapman voted for Hamilton-Smith in his successful bid as deputy leader on 31 March 2010 in a vote held after the third consecutive Liberal loss at the 2010 election where Chapman gained a substantial 9.1-point two-party swing. Voting for Hamilton-Smith as deputy meant not voting for Evans.^[5] Chapman drew headlines in the last week before the 2010 election for not being willing to publicly refuse challenging Redmond for the leadership and faced accusations, particularly by Hamilton-Smith, of derailing the Liberal campaign, with "Chapman Could Challenge" posters hung beneath many of the Liberal "Redmond is Ready" posters.^[6]

Chapman was reappointed deputy opposition leader on 4 February 2013, and chose to announce she would rule out challenging new leader Steven Marshall.^[7]

Upon the fourth consecutive Liberal loss at the 2014 election, Chapman suffered a 1.5-point two-party swing but still retained Bragg with a comfortably safe 68.7 percent two-party vote. After the Liberal party won the election at the 2018 election, she was sworn in as Deputy Premier of South Australia.

2020s

Chapman was additionally appointed Minister for Planning and Local Government on 29 July 2020.^[8] Chapman lost a vote of non-confidence as Deputy Premier in the South Australian Parliament on 18 November 2021 after a parliamentary inquiry found her conflict of interest as Minister for Planning and Local Government and recommended for her to be found guilty of contempt of parliament for misleading the house.^[9] On 22 November 2021, she resigned as Deputy Premier and Minister for Planning and Local Government, and stepped down as Attorney-General while officially still holding the role and remaining in cabinet.^[10][11] On 3 May 2022, the Ombudsman's Report into the allegations exonerated Chapman of all wrongdoing, finding she had no conflict of interest.^[12]

Chapman retained her seat in parliament at the 2022 South Australian state election, despite a swing against her party which resulted in a change of government.^[13] A month after the election, Chapman announced her intent to resign from parliament. She officially resigned on 31 May 2022, with her successor to be elected in a by-election on 2 July 2022.^[14][15][16]