Reid was born in Croydon, London to an English father, of Scottish ancestry, and an English mother.
Reid was educated at the independent Croham Hurst School in Croydon (from 1975—81) with the comedienne Sue Perkins, followed by the independent Croydon High School (1981—87) and St Paul's Girls' School (1987—89) in London before studying Politics, Philosophy and Law at the University of Bristol (1989—92). She was Editor of EpiGram, the student newspaper there, when it was shortlisted for Best Student Newspaper in The Guardian/NUS student journalism awards 1991. Reid also has a Postgraduate Diploma in Broadcast Journalism from Cardiff School of Journalism.
Reid started her career at BBC Radio Bristol and then become a reporter for Radio 5 Live, as well as latterly a producer.
She then joined BBC News 24 where she spent two years as a reporter, and then presenter. Reid then spent three months doing charity work in Sri Lanka, before becoming a reporter for Breakfast News. After a maternity break, Reid became a presenter on the BBC News channel.
Reid is one of the main presenters on BBC Breakfast, presenting alongside Charlie Stayt and previously Bill Turnbull on Fridays, Saturdays and Sundays. She has held her current role since 2004, when she replaced Mishal Husain, who went on maternity leave and can occasionally be seen on Breakfast during the week. In 2010 she stepped down from presenting Breakfast on Saturdays and Sundays to take a role on new programme Sunday Morning Live.
She was also the regular newsreader during the headlines on The Andrew Marr Show, On 16 May 2010 she stood in briefly for Andrew Marr, for the Sunday newspaper review, when he arrived late for the programme after interviewing the new Prime Minister David Cameron. This led to a slightly awkward scene when he turned up halfway through and there was a confusion as to who was fronting the show.
On 6 February 2009, Reid made a swift, off-the-cuff apology, while hosting Breakfast with Charlie Stayt, when a wrongly-cued, uncut audio clip of Hollywood actor Christian Bale, featuring the shouted line "You don't fucking understand!" was played on air. This was broadcast live at 6.55am on BBC1 in the UK. The clip was cut immediately and Susanna's gasps were audible during and immediately after the clip.
On 22 February she presented the BBC's live coverage of the 2009 Oscars from Los Angeles. She also presented coverage of the 2010 Oscars on 7 March.
Reid has also graced the stage and screen as an actress, appearing with Shirley-Anne Field in Agatha Christie's Spider's Web in 1982, and then in 1985 alongside Harriet Walter, Peter Barkworth, Adrian Dunbar and Derek Thompson in 'The Price' on Channel 4.
Reid is heavily involved with charities. She is a regular contributor to Media Trust, the charity which links other charities to the media industry, and has hosted events for the Myotubular Trust and Voluntary Arts England. In 1998 she worked for three months in Sri Lanka as a voluntary media consultant for a charity which counsels victims of the civil war and operates orphanages and social development programmes.