Isobel Baillie was born in Hawick, Scottish Borders, in 1895. She worked in a music shop and as a clerk at Manchester Town Hall, and made her orchestral debut with the Hallé Orchestra in 1921 under the name Bella Baillie, having already appeared in several Manchester chamber concerts series. After studies in Milan, she won immediate success in her opening season in London in 1923. Her favourite work was Handel's Messiah, of which she gave over 1,000 performances during her career. She was often in demand for choral works; apart from Messiah, she was noted in Haydn's The Creation, Mendelssohn's Elijah, and Brahms's A German Requiem. In 1933 she became the first British performer to sing in the Hollywood Bowl in California. In 1937 Arturo Toscanini chose her to sing Brahms' Requiem. 
Her performances in Gluck's Orpheus (always in English) and Gounod's Faust were very popular. However, her strength was in British music, including Vaughan Williams' Serenade to Music (of which she was one of the original singers) and Elgar's The Kingdom.
With the exception of 1933, she sang at the Three Choirs Festival every year from 1929 to 1955. Miss Baillie sang 'Messiah' for the Halle Orchestra annually for twenty-six consecutive seasons and for the Royal Choral Society at the Royal Albert Hall on thirty-three occasions. In all she sang this work for over fifty years.
She taught at the Royal College of Music (1955-57, 1961-64), Cornell University (1960-61) and the Manchester School of Music (from 1960). She sang with Kathleen Ferrier on the occasion of Ferrier's first complete performanc of 'Messiah'. They often sang together in that work and others subsequently.
She made her first test recording for HMV in 1924, but nothing came of this. However, she made her first commercially released recordings for Columbia in 1925 and her last, at the age of 79, in 1974.
She is noted for her advice to aspiring singers: "Never sing louder than lovely".
Scottish baritone singer Calum A. Macdonald is related to Isobel Baillie.
- Kennedy, Michael (2006), The Oxford Dictionary of Music p. 48
- Christopher Fifield, Ibbs and Tillett: The Rise and Fall of a Musical Empire
- Kennedy, Michael (2006), The Oxford Dictionary of Music p 48
- D. Brook, Singers of Today (Revised Edition - Rockliff, London 1958), pp. 27-32.
- Kennedy, Michael (2006), The Oxford Dictionary of Music, 985 pages, ISBN 0-19-861459-4
- Isobel Baillie and Bryan Crimp, 'Never Sing Louder Than Lovely' (Hutchinson 1982)