Early political career: 1990–2001

While a student at Paisley College of Technology he was deputy president of the student union. Rennie ran the Scottish Young Liberal Democrats (later reformed as Liberal Youth Scotland) and after graduation went on to work for the English Liberal Democrats in Cornwall. He then went on to work for the Liberal Democrats' campaigns department, and was the successful agent in the 1993 Christchurch by-election in Dorset.

After managing the party's campaigns in the South West England region in the 1997 General Election, he moved back to Scotland where he was Chief Executive of the Scottish Liberal Democrats from 1997 to 1999, and then the party's Chief of Staff in the new Scottish Parliament from 1999–2001.

McEwan Purvis: 2001–2006

From 2001 to 2006 he worked for the small Scottish communications firm McEwan Purvis as an account director helping advise businesses and charities such as the Royal Society of Chemistry and Asthma UK. During his time at McEwan Purvis, Rennie was a press adviser to Fife Council's Liberal Democrat Opposition Group and a member of the Dunfermline Focus editorial team, working with Dunfermline's Lib Dem councillors on local issues.

Member of Parliament: 2006–2010

After Labour MP Rachel Squire died, Rennie stood in the Dunfermline and West Fife by-election on 9 February 2006 and overturned a Labour majority of 11,800 to win the seat with a swing to his party of 16.24%.^[6] In the House of Commons, he was a member of the Liberal Democrat defence spokesperson team, chair of their parliamentary campaigns unit, and a member of the Commons Defence Select Committee.

During his time as an MP, he campaigned on local constituency issues such as abolishing the bridge tolls, banning sex offenders from being driving instructors in 2008,^[7] improvements to cancer services at Queen Margaret Hospital, and local jobs (including at Longannet Power Station and Rosyth Dockyard).

In the General Election of 6 May 2010, Rennie lost his seat to the Labour candidate Thomas Docherty.

Special Adviser and consultant: 2010–2011

He was then for a time Special Adviser to the new Liberal Democrat Scottish Secretary Michael Moore MP.

Elected MSP and Leader of the Scottish Liberal Democrats: since 2011

Rennie returned to front-line politics as an MSP when he won a regional list seat for the Liberal Democrats in the Scottish Parliament's Mid Scotland and Fife region at the Holyrood elections on 5 May 2011. He was the only new Liberal Democrat MSP to win a seat in this election.

Following the resignation of leader Tavish Scott, Rennie stood in the resulting leadership election. He was the only candidate to be nominated, and was declared elected when the nominations closed on 17 May. He vowed to stand up to the "SNP bulldozer" majority, and refused to distance his party from the UK Liberal Democrats.^[8]

During the 2014 Scottish independence referendum, Rennie campaigned alongside Better Together to remain in the United Kingdom.^[9] He opposes a second independence referendum, favouring a more federal UK.^[10][11]

In the 2016 Scottish Parliament election, the Liberal Democrats again elected five MSPs, while gaining two constituency seats and holding their existing two with increased majorities. Rennie was elected in North East Fife, gaining it from the SNP with a 9.5% swing.

Rennie was re-elected in North East Fife for the 2021 Scottish Parliament election,^[12] with doubled majority.^[13] On 12 July 2021, Rennie announced his resignation as leader of the Scottish Liberal Democrats, effective from September.^[14] He was replaced by Alex Cole-Hamilton, the MSP for Edinburgh Western.^[15]