He was first elected as a Labour councillor in 1988. He held a number of positions within the party in Edinburgh including Group Secretary, Whip and Deputy Lord Provost. He was also involved in the trade union movement serving on the national executive of the Civil and Public Services Association (CPSA).
In late October 2005 he resigned from the Labour Party and joined the SNP. He said he had come to the conclusion that independence was in Scotland's interests and that Tony Blair's policies no longer represented the Labour Party he had joined. He found himself ostracised by his former colleagues.
He would be the sole SNP councillor on City of Edinburgh Council until the 2007 Scottish local elections. He was elected in the Forth ward under the new proportional representation system topping the poll with 2,472 first preferences and exceeding the quota. After the elections the Scottish Liberal Democrats formed a coalition with the SNP to run the Council and Cardownie became Deputy Leader.
He was re-selected as a candidate for the 2012 Scottish local elections again in the Forth ward this time with a running-mate. He polled 1,377 first preferences and took the third seat in the ward exceeding the quota. After the elections he was re-elected Leader of an enlarged SNP group and he again became Deputy Leader of the City of Edinburgh Council as the Scottish Labour Party and the SNP formed a grand-coalition after the election. In March 2015 he stood down as leader of the SNP group.
- "Cardownie defects to SNP and launches attack on Labour policy". Edinburgh Evening News. 25 October 2005. Retrieved 17 October 2015.
- "Leading councillor defects to SNP". BBC News. 25 October 2005.
- Gordon, Tom (5 January 2014). "SNP council boss: a one-time union militant, says confidential memo by top civil servant". Sunday Herald. Retrieved 17 October 2015.
- Cardownie, Steve (27 October 2005). "So much Labour and yet so little in return". The Scotsman. Retrieved 17 October 2015.
- Lorimer, Kerry (27 May 2011). "Interview: Steve Cardownie". Holyrood. Retrieved 17 October 2015.
- Ferguson, Brian (10 November 2011). "Crisis? What crisis? say 'odd couple' who hold key to Edinburgh's future". The Scotsman. Retrieved 17 October 2015.
- "Steve Cardownie says it's SNP's time in the driving seat". Edinburgh Evening News. 27 April 2012. Retrieved 17 October 2015.
- Carswell, Clare (30 March 2012). "Who are the Edinburgh candidates for council elections 2012?". Edinburgh Evening News. Retrieved 17 October 2015.
- "Labour and the SNP reach agreement to form Edinburgh council coalition". STV News. 7 May 2012. Retrieved 17 October 2015.
- "SNP group leader Steve Cardownie steps down". BBC News. 11 March 2015.
- "Steve Cardownie to become Deputy Lord Provost". Edinburgh Evening News. 27 May 2015.
- Swanson, Ian (2 September 2016). "Deputy Lord Provost, Steve Cardownie, to quit the City Council". Edinburgh Evening News. Retrieved 10 September 2016.
- Steve Cardownie Council page