Boyle was born in Blackburn, West Lothian to Patrick, a storeman at the British Leyland factory in Bathgate, and Bridget Boyle, a shorthand typist. The youngest of four brothers and five sisters, Boyle was born when her mother was 47. The Sunday Times writes that it was a difficult birth, during which Boyle was briefly deprived of oxygen, leading to mild brain damage. She was diagnosed as having learning difficulties, which led to bullying and mockery at school.

After leaving school with few qualifications, she was employed in the kitchen of West Lothian College, and took part in government training schemes. She would visit the theatre from time to time to listen to professional singers, and in 1995, she auditioned for Michael Barrymore's My Kind of People, which was looking for contestants at the Braehead Shopping Centre in Glasgow, but she said she was too nervous to make a good impression. She took singing lessons from a voice coach, Fred O'Neil, and in 1999 she made her only previous recording, singing Cry Me a River for a charity CD funded by the local council to commemorate the Millennium.

Boyle's father died in the 1990s, and her siblings had left home, leaving Boyle to look after her aging mother, who died in 2007 at the age of 91. Boyle still lives in the family home, a four-bedroom council house, with her ten-year-old cat, Pebbles. Her mother had always encouraged her to enter local singing competitions, which she won several times, and tried to persuade her daughter to enter Britain's Got Talent, urging her to take the risk of singing in front of an audience larger than her parish church. Boyle has said she didn't feel ready to do it until after her mother's death. Her performance on the show was the first time she had sung since then.

Boyle is currently unemployed, and active as a volunteer with Our Lady of Lourdes, a church in Whitburn. She has never married, and during an interview just before she sang on the talent show, she said she had also "never been kissed," but added later, "Oh, I was just joking around. It was just banter and it has been blown way out of proportion."