McDonnell's first involvement with politics came when he joined the National Democrats and stood as the party candidate in the 1970 election in North Antrim and lost to Ian Paisley. McDonnell first won election to Belfast City Council in 1977, representing Belfast "Area A" which included the Short Strand and Upper Ormeau areas. He lost his council seat in a surprise result in 1981 but returned in 1985 and served as the first Catholic Deputy Mayor of Belfast in 1995–96. He first stood for the Westminsterconstituency of South Belfast in the 1979 general election and subsequently contested the constituency at each subsequent general election, though not in the 1986 by-election. He was also elected from the constituency to the Northern Ireland Peace Forum in 1996 and the Northern Ireland Assembly in 1998 and 2003. 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 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. In a 2012 interview with The News Letter, McDonnell criticised Sinn Féin. He said the party were 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. 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 to 600, as "a bureaucratic numbers game initiated by the Tories for purely party political advantage". In June 2013, the SDLP abstained during the vote on the Civil Service Bill 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. Despite the reported claims 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. 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. On 9 June 2017, McDonnell lost his South Belfast seat to Emma Little-Pengelly in the 2017 general election.
