She was born in Grangemouth, central Scotland, in 1952. Her mother, Zetta, sang in folk clubs and wrote songs. The family moved to Aberdeen, where she sang with the Aberdeen Folk Singing Club from the age of 10. By the age of 12 she had appeared on television in My Kind of Folk. When her parents divorced, Isla adopted her mother’s maiden name, Sinclair, adapted slightly.
By 1971 St Clair was voted Folk Singer of the Year by the New Musical Express.
In the mid-70s, St Clair presented a children’s programme on Grampian TV called Isla’s Island. But she rose to prominence when in 1978 she became the assistant to game show host Larry Grayson in the hugely successful BBC show The Generation Game. Her Scottish accent was an accompaniment to Grayson’s camp ambivalence.
From the mid-80s to the early 90s, St Clair disappeared from the public eye while she raised a family. Then, starting with Inheritance in 1993, she produced several albums devoted to Scottish folk songs. In 1997 she had a series on BBC Radio 2, Tatties and Herrin’, devoted to songs from the North-East of Scotland. She has recorded many of the Child Ballads. In 2003 she released My Generation, a collection of children’s songs, many of them remembered from her own time in the playground. St Clair’s most recent album is called Looking Forward to the Past.
In 2003 she appeared in, and co-produced, a documentary film called When the Pipers Play, about the Great Highland Bagpipe. In the same year, she was awarded an honorary degree as a Master of the University of Aberdeen for her lifelong contribution to the traditional music of Scotland.