Rau was admitted as a solicitor and barrister of the Supreme Court of South Australia in 1981. He worked as an adviser to Hawke government ministers Mick Young, Michael Tate and Neal Blewett from 1985 to 1988. He has also served on the ALP State and National Executives.
After concluding his service as a political adviser, Rau worked as a solicitor at Duncan & Hannon and later as a solicitor and barrister at the firm of Johnston and Withers. After becoming a partner at Johnston & Withers, Rau established a private legal practice in Adelaide, specialising in industrial, employment, commercial and personal injuries law. In 1998, he was admitted to the independent bar in South Australia, practising as a member of Murray Chambers.
Rau has appeared as counsel 49 times before the South Australian Industrial Relations Commission, 22 times before the South Australian Industrial Relations Court, 15 times before the Supreme Court of South Australia and 7 times before the Federal Court of Australia (including once in his capacity as Attorney-General).
His first political experience as a Labor candidate occurred at the 1993 federal election, when he stood for the Division of Hindmarsh, where he was narrowly defeated by Liberal Party candidate Christine Gallus by 1.6 percent.
Prior to the 2002 state election, Rau contested Labor preselection for the safe seat of Enfield. The seat had previously been Ross Smith, held by Ralph Clarke, who had recently been deposed as the party's deputy leader. The local party branch chose Clarke who received 60 of 74 votes. However, the party's state executive stepped in and installed Rau as the pre-selected candidate. Clarke ran as an independent Labor candidate, receiving a respectable 23 percent of the vote; however, Clarke narrowly fell short of overtaking Rau and winning the seat on Liberal preferences. Rau easily won the seat with a 35.9 percent primary and 65.9 percent two-party vote. He is aligned with Labor's right faction.
Rau gained publicity in 2004 over his involvement in the Real Estate Industry – Reform bill, which was designed in an attempt to stop industry practices such as dummy bidding at auctions.
The 2006 state election saw Rau retain Enfield with a 63.4 percent primary and 74.5 percent two-party vote. At the 2010 state election, Rau suffered a swing to finish with a 52.6 percent primary and 60.5 percent two-party vote.
Rau became Attorney-General when Michael Atkinson stepped down from the position following the 2010 election. Like his predecessor, Rau has also been described as a social conservative.
Rau expressed approval for the introduction of an R18+ video games classification following the resignation of Atkinson. The issue has been one for which his predecessor Atkinson received significant media attention. Rau appears to be taking a different view to his predecessor and is considering allowing an introduction of an R18+ classification.
In February 2011, Rau was elevated to Deputy Premier following the resignation of Kevin Foley from the position.
On 22 November 2016, Rau was appointed a Senior Counsel by the Supreme Court of South Australia.
In addition to Deputy Premier and Attorney-General, Rau represents the following portfolios in the Cabinet of South Australia:Minister for Justice Reform
Minister for Planning
Minister for Industrial Relations
Minister for Child Protection Reform
Minister for the Public Sector
Minister for Consumer and Business Services
Minister for City of Adelaide
Rau attended Henley High School in Henley Beach and lives in the same suburb. He was married to Anna Rau, councillor for the City of Charles Sturt. Rau enjoys cooking, gardening and reading non-fiction.