Soon after college graduation in 1840 Bain began to contribute to The Westminster Review, thus becoming acquainted with the philosopher John Stuart Mill and his circle in London. There Bain served as secretary of the board of health (1848–50) and for the next 10 years was variously employed in the civil service and as an educator. From 1860 to 1880 he taught logic and English literature at the University of Aberdeen, where he advocated the reform of teaching methods in Scotland. During this period he wrote several books on grammar and rhetoric and a two-volume work on Logic (1870) containing a detailed account of the application of logic to the natural sciences. He also devoted himself to the study of psychology, adopting a rigorously scientific approach. Bain sought to find physical correlatives for such abstract concepts as “idea” and “mind” and stressed the need for further investigation of the processes of the brain and the nervous system.

Bain founded the first journal devoted to psychology, Mind, in 1876. Among his works in psychological theory are On the Study of Character (1861), Mental and Moral Science: A Compendium of Psychology and Ethics (1868), and Mind and Body: The Theories of Their Relation (1873). His other writings include John Stuart Mill: A Criticism, with Personal Recollections (1882) and an Autobiography (1904).