Son of Free Church Minister John Baillie, he was born in the Free Church Manse in Gairloch, Wester Ross, on 26 March 1886. A leading theologian, he held academic posts in the UK, USA and Canada. His brother Donald Macpherson Baillie was Professor of Systematic Theology at the University of St. Andrews and his other brother Peter Baillie served as a missionary doctor at Jalna, India.
He met Florence Jewel Fowler while he was in France during the First World War. They married at Leamington Spa in 1919. Their only child, Ian Fowler Baillie, was born in 1921.
Baillie wrote A Diary of Private Prayer (1936), regarded as a devotional classic. But his most important contribution to theology was an exploration of the relationship between the knowledge of God to spiritual and moral experience.