In 2004 he became his party's deputy leader. In the 2005 general election McDonnell generated one of the most sensational results in Northern Ireland when he won South Belfast, primarily due to a split in the unionist vote. He received 10,339 votes while the Democratic Unionist Party (DUP) candidate Jimmy Spratt received 9,104 votes and Ulster Unionist Party candidate Michael McGimpsey received 7,263 votes. He was then re-elected by an increased majority in the 2010 general election. On 5 November 2011, he was elected leader of the SDLP at its conference in Belfast, succeeding Margaret Ritchie.[2]
In a 2012 interview with The News Letter, McDonnell criticised Sinn Féin. He said the party was run along "Soviet style" lines where there was a military structure and where former terrorists were being placed into positions of power. He also claimed many people voting for Sinn Féin were doing so as an act of defiance.[4]
As SDLP chief, McDonnell described the terms of the Sixth Periodic Review of Westminster constituencies, a seemingly blocked plan to reduce the number of MPs in the House of Commons by 50 (including two from Northern Ireland) to 600, as "a bureaucratic numbers game initiated by the Tories for purely party political advantage".[5]
In June 2013, the SDLP abstained during the vote on the Civil Service (Special Advisers) Bill[6] in Stormont, ensuring its passing. This led to claims from Sinn Féin that the SDLP was endorsing a "hierarchy of victims"' agenda and abandoning the principles of the Good Friday Agreement.[7]
Despite the reported claims[7] from Sinn Féin that it was inevitable that someone would mount a legal challenge to "what republicans view as a discriminatory law" no such challenge has since emerged. In the 2015 United Kingdom general election he stood again in Belfast South and was returned on 24.5% of the vote, the lowest ever vote share recorded by a successful MP in any part of the UK.[8]
On 14 November 2015, McDonnell lost the leadership contest held at the SDLP's annual conference. His successor as leader of the party, Colum Eastwood, won with 172 votes to the 133 that McDonnell received.[9]
