He served as Treasurer of South Australia and minister for other portfolios in the Weatherill cabinet between 2011 and 2018.

As a student at the University of Adelaide he became involved in Australian Young Labor, drove taxis and went on to run a small business before[citation needed] becoming an industrial officer for the Shop, Distributive and Allied Employees Association (SDA) in the 1990s.

In 2009, he was appointed to cabinet;[2] Koutsantonis has served in a range of ministerial portfolios with responsibility for finance, state development, mineral resources and energy, small business, ministerial resources and energy, and for road safety,[3] where Koutsantonis was forced to apologise for his "unacceptable" driving record which listed 58 traffic offences and over $10,000 in fines.

[4] Following the retirement of Liberal's Rob Lucas at the 2022 state election, Koutsantonis became the longest serving member of the South Australian Parliament.

[5] His long-serving parliamentary service was recognised by Labor leader Peter Malinauskas in his victory speech on election night.