McDonald was Deputy Leader of Sinn Féin before taking over from former leader Gerry Adams in February 2018. She was first elected to the Dáil in 2011. She was a member of the Public Accounts Committee between 2011 and 2017. She has served on the Joint Oireachtas committees on Public Expenditure and Reform and the Future of Mental Health. She was an MEP for Dublin from 2004 to 2009.