McDonagh is a Roman Catholic of Irish descent. McDonagh was educated at the Holy Cross School, New Malden and later studied Politics at the University of Essex. She was a clerical officer for the DHSS between 1981–83, a receptionist at the Wandsworth Homeless Persons Unit from 1984–86, and a housing adviser from 1986–88. Prior to being elected to Parliament she worked as a Development Manager for Battersea Churches Housing Trust from 1988–97. She also served as a councillor on London Borough of Merton for Colliers Wood ward between 1982 and 1998, chairing the Housing Committee between 1990 and 1995, being instrumental in the rebuilding of Phipps Bridge Estate.
In March 2003, McDonagh voted in favour of the country going to war with Iraq. She has consistently voted against any inquiry into the Iraq War. In December 2015, she was among the minority of Labour MPs who voted in favour of extending UK military airstrikes against ISIL into Syria. She has written that it was a decision "not easy to come to". McDonagh abstained from a vote about the UK's support for Saudi Arabia's military campaign in Yemen. The vote was defeated by a majority of 90. As noted by commentators, the vote would have succeeded if 97 Labour MPs had not abstained.
Expenses
In April 2000, her office sent a party politicalquestionnaire to 200 of her constituents using parliamentary resources; a spokesman for McDonagh subsequently said it was a "mistake". McDonagh promised to apologise and reimburse the cost to her office. In 2007, her expenditure on stationery and postage attracted criticism, being more than any other MP's for postage from 2003 to 2006. In total, her office spent £126,833 on postage in the four-year period, an average of almost £32,000 per year. When adding in stationery costs, the expenditure was approximately £50,000 in both 2004–05 and 2006–07. McDonagh responded stating, "I believe the job of an MP is to keep in contact with constituents on important issues."
Mobile phone theft
In October 2010, her mobile phone was stolen from her car. Although not implicated in the robbery itself, it became evident that The Sun newspaper had accessed the phone, including messages stored on it. She sued the paper and in March 2013 won "substantial damages."
Antisemitism comments
In 2019, McDonagh was criticised by some left-wing members of the party after she appeared to agree with a statement put forward by John Humphrys on BBC Radio 4's Today programme. McDonagh said anti-semitism is a problem in the Labour Party, because "part of politics, of hard left politics, to be against capitalists and to see Jewish people as the financiers of capital." When Humphrys asked her if that meant that "to be anti-capitalist you have to be antisemitic", McDonagh replied, "Yes."
