While at university, she became involved with the leadership of the Adelaide University Labor Club in 1988, and has been a delegate to the South Australian Labor Party State Convention every year since 1989, (with the exception of 1995). She also worked part-time for the Construction, Forestry, Mining and Energy Union (CFMEU), and won a position on the National Executive of the National Union of Students. A number of her contemporaries at university went on to become Australian politicians. Former senator for South Australia, Natasha Stott Despoja, was a contemporary, along with current Premier of South Australia Jay Weatherill (with whom she had a relationship) and Mark Butler, Labor MP for Port Adelaide. Wong graduated from the University of South Australia in 1992, and continued her association with the CFMEU as an industrial officer. She was admitted to the South Australian Bar in 1993.

During 1995 and 1996, Wong acted as an advisor to the CFMEU and to the newly elected New South Wales state government, specialising in the area of forest policy in the middle of the fierce 1990s environmental battles over logging in NSW.

During her legal career (1996–2002), Wong appeared as counsel in 11 reported decisions of the Australian Industrial Relations Commission, 15 reported decisions of the South Australian Industrial Relations Court, 8 reported decisions of the South Australian Industrial Relations Commission, 3 reported decisions of the South Australian Workers Compensation Appeal Tribunal and 10 reported decisions of the South Australian Workers Compensation Tribunal.

In 2002, her brother Toby took his own life ten days after her election to the Senate. As of 2007, Wong's father lives overseas, but they keep in touch.

In June 2005, Wong was appointed Shadow Minister for Employment and Workforce Participation, and Shadow Minister for Corporate Governance and Responsibility. Following the reshuffle in December 2006, she became responsible for the portfolios of Public Administration and Accountability, Corporate Governance and Responsibility, and Workforce Participation.

In December 2007, in the wake of the Labor Party victory in the 2007 election, Wong was appointed Minister for Climate Change and Water. She accompanied then Prime Minister Kevin Rudd to Bali for the international climate change talks. Wong led final negotiations as Chair of the United Nations Working Group in the closing days of the United Nations Climate Change Conference in December 2007, shortly after her appointment as minister.

In September 2010, Prime Minister Julia Gillard promoted her to succeed Lindsay Tanner as Minister for Finance and Deregulation.

Wong's domestic partner, Sophie Allouache, is a public servant and former University of Adelaide Students' Association President. In December 2011, Allouache gave birth to their first child, after announcing the IVF-assisted pregnancy using donor sperm in August 2011. Allouache gave birth to their second daughter in 2015, at the Adelaide Women's and Children's Hospital.

In February 2013, Wong was appointed Deputy Leader of the Government in the Senate, following the resignation of Chris Evans, the then Government Leader in the Senate. In June 2013, following Stephen Conroy's resignation, she became the first female to be appointed Leader of the Government in the Senate. Following Labor's defeat at the 2013 Australian federal election, Wong was appointed the Leader of the Opposition in the Senate, becoming the first woman to do so.