She went to Westminster School before university. After graduation, she worked as a business journalist for two years, including a traineeship at EuroWeek magazine. She then joined the Liberal Democrats in their press team, becoming in 1997 press secretary and advisor to then leader Paddy Ashdown.
After Ashdown stepped down as leader in August 1999, Green joined the BBC for a short time to work on On The Record with John Humphrys, then became a journalist and columnist at the Financial Times. Employed first on the home news desk, she was then deputy world news editor, then the paper's education correspondent, and finally political correspondent.
After giving birth to her first child in 2009, she has since been freelance, working for The Times, The Observer, The Sunday Times and Intelligent Life. She has also appeared as a pundit and commentator on BBC Radio 4 The World Tonight, BBC World Service Newshour; BBC1 The Politics Show and This Week; BBC Radio 5Live, LBC and BSkyB. She has also appeared on Newsnight.