Williams was born in London to Welsh parents, and raised in Eastbourne, East Sussex.Her family hailed from Llanelli, South Wales. She gained a BA in English and History from Oxford Polytechnic, and was also a student of Japanese and international business at Liverpool John Moores University.

Prior to her career in television journalism, Williams spent over a decade with BBC Radio after joining the corporation in 1985. She worked on BBC Local Radio in Sheffield, Leeds and Manchester before becoming a producer and reporter in Liverpool, and later joining a national spoken word network, BBC Radio 4, as a producer on The World at One and PM programmes. Williams spent the next few years reporting and editing news programmes on the station, and also on BBC Radio 5 Live.

Williams' first job in television news was when she joined digital rolling news channel BBC News 24, which was soon to launch, as an output editor. During screen tests for potential presenters, one applicant became unwell and Williams was asked if she would be willing to audition for the role.

When she did, producers were so impressed that they offered her the position alongside Gavin Esler presenting from 4pm—7pm; one of the prime slots on the channel.

She remained with the channel for nearly two years before joining the Six O'Clock News in 1999 as their Special Correspondent. During this time she also began filling in for the programme's presenters, Huw Edwards and Fiona Bruce and during 2001 to 2002 Sian presented the Six O'Clock News on Fridays while Fiona Bruce was on maternity leave, as well as regularly presenting the national news bulletins on BBC One at weekends.

She joined the BBC's Breakfast programme in January 2001 as a relief presenter, initially presenting on Friday-Sunday alongside Darren Jordon, to cover for main presenter, Sarah Montague, and then later with Jeremy Bowen, to cover for Sophie Raworth.She also regularly deputised on the Six O'Clock News and the One O'Clock News during this period.In 2004, Sian covered for Raworth on the Six O'Clock News during her maternity leave, co-presenting with George Alagiah, and the following year, reported from Sri Lanka and Thailand on the 2004 Indian Ocean earthquake and from Pakistan on the Kashmir earthquake.

In May 2005 she was confirmed as the main female presenter of the programme, presenting initially alongside Dermot Murnaghan and since January 2008, with Bill Turnbull on Mondays to Thursdays.

Other television credits include BBC Wales (One Show, with Aled Jones, and the Big Welsh Challenge, where she spent a year learning Welsh) as well as presenting BBC One daytime programmes including Now You're Talking and City Hospital. In 2010, Williams reported for Watchdog.

Sian was the President of TRIC (Television and Radio Industries Club) for 2008-09. She supported two charities during her year of office.

Williams married Neale Hunt in February 1991, a former director of advertising firm McCann Erickson, London. The couple have two sons, and were divorced in 2001.

Williams married Paul Woolwich in early 2006.Williams' third son was born in October 2006, for which she disclosed in an interview to promote blood donation that she received two litres of blood as a result of complications from the birth. Williams gave birth to a daughter, Eve, in March 2009.