Baillie was born on 15 January 1964 in Hong Kong to Sophie and Frank Barnes. Her mother is Scottish and her father Portuguese. She attended the private St Anne's School, Windermere in the Lake District. She studied at Cumbernauld College, Strathclyde University and the University of Glasgow.
She married Stephen Baillie in 1982. Baillie lives in Dumbarton with her daughter.
Baillie has been a resource centre manager at Strathkelvin District Council and a community economic development manager at East Dunbartonshire Council. She was chair of the Scottish Labour Party in 1997.
She was first elected at the inaugural election for the Scottish Parliament in May 1999. She was re-elected in 2003. A member of the Scottish Parliament's Justice 2 Committee and Public Petitions Committee, she was previously a member of the Scottish Executive, serving as Minister for Social Justice when Henry McLeish was First Minister, during which time she was involved with the Homelessness Task Force.
As a backbench MSP she has campaigned for a public inquiry into a lethal outbreak of Clostridium difficile at the Vale of Leven Hospital. In 2007 she defended Wendy Alexander on Newsnight Scotland during the controversy regarding illegal donations to Alexander's leadership campaign.
In 2009 she successfully brought into being an act of the Scottish Parliament, with the unanimous support of all MSPs, to allow for greater protection of disabled parking spaces.
She has opposed minimum pricing of alcohol, being unconvinced about the overall benefits, and stating that it was not the best way of tackling the country's alcohol-related problems but has backed a tax-based alternative amongst other measures.
Baillie held the position of Shadow Health Minister in the Shadow Cabinet of Iain Gray, retaining that post in December 2011 following the election of Johann Lamont as Gray's successor. When Lamont announced a major shakeup of the Labour frontbench team on 28 June 2013, Baillie was moved from Health to Social Justice and Welfare.
On 27 March 2014, she stood in for Lamont at First Minister's Questions while Lamont was attending the funeral of veteran Labour politician Tony Benn. She also stood in at FMQs following Lamont's resignation as Labour leader in October 2014. Baillie ruled herself out of standing in the leadership election that followed Lamont's departure, stating that she wanted a "supporting role" rather than to be Labour leader.
She has campaigned on behalf of the families of patients affected by an outbreak of Clostridium difficile colitis at Vale of Leven Hospital in her constituency. The inquiry into the outbreak cost £10 million, while the families were offered £1m, something that prompted Baillie to make an emotional plea to Health Secretary Shona Robison during a session of the Scottish Parliament in November 2014 during which she pressed for greater compensation for those affected.