Born in Guildford, Surrey, Newman is a daughter of two chemistry teachers, and was educated at Charterhouse, an independent school. She was on the path to a career as a violinist or in the legal profession before changing her plans as a result of seeing BBC journalist Kate Adie on television. Newman graduated with a first from Oxford University, where she read English at Lady Margaret Hall.
After university, Newman briefly worked for Media Week and The Independent (as business correspondent) before joining the Financial Times at the age of 23. Her older colleague Alice Rawsthorn acted as a mentor at the FT, where Newman worked as a media and then (for three years) political correspondent. In 2000, Newman gained a Laurence Stern fellowship to work at The Washington Post for four months. During her period in the US, she followed the 2000 Presidential campaign of Green Party candidate Ralph Nader.
She joined Channel 4 News as a political correspondent (and deputy to political editor Gary Gibbon) in January 2006. In this role she broke several stories, including claims the Treasury pushed through the nomination of then Chancellor Gordon Brown's close friend Ronald Cohen for the House of Lords, challenging Peter Mandelson at the Brighton Labour Party conference in 2009, over his claimed use of the C-word in a conversation with Rebekah Brooks, then Wade, the CEO of News International.
Newman became the first female co-presenter of the programme in 2011. Alongside this, she has also headed the team behind the FactCheck blog.
From 2013 to 2015, Newman's pursuit of a story about the allegations of improper conduct levelled at Lord Rennard, once a leading figure in the Liberal Democrats, included her participation in an LBC local London radio phone-in on 27 February 2013 to quiz deputy Prime Minister Nick Clegg on the issue. Newman, a feminist, has commented that sexism was endemic at Westminster during her period as a lobby correspondent there, but has also said that the newspaper industry is even worse. She told Natasha Lunn in an interview for Red magazine in 2016: "As a woman in the media I feel a duty to make sure we report those issues. I’ve always wanted to right injustices; I suppose what’s changed is I’ve now got a keener sense of how journalists can hold power to account".
In late 2013, she travelled with William Hague and Angelina Jolie to highlight the effect of warzone rape in the Democratic Republic of Congo. Newman also investigated the balance of the curriculum in some faith schools in the UK for the Dispatches series on Channel 4.
A regular commentator on politics in other media outlets, Newman has appeared as a guest panellist on Have I Got News For You and blogs for The Daily Telegraph and Economia magazine.
Newman was long-listed for the Orwell Prize (Journalism) in 2010 and again in 2011 for the blog prize. She was announced as one of the judges for the Baileys Women's Prize for Fiction in 2015.
On 1 February 2015 Newman tweeted that she was "ushered onto the street" when she went to South London Islamic Centre (SLIC) for a 'Visit My Mosque' programme in error. A spokesperson for the Hyderi Islamic Centre had said Newman had simply visited the wrong address. Later, The Huffington Post obtained CCTV footage which, "appeared to contradict" her account; Newman has since apologised.
Newman married writer John O'Connell, whom she met at university, in 2001. The couple have two daughters. Newman has talked openly about deciding to abort another child, after discovering 13 weeks into her pregnancy that the baby had a very rare condition which meant there was a high chance it would die before reaching full term, or during labour.