James Douglas (Jim) Pearson was born on 17 December 1911 in Cambridge but brought up as an only child in Linton by foster parents who both suffered from deafness, which perhaps encouraged his love of books. He was educated at Cambridge County High School for Boys but left school at the age of sixteen with, as he put it, ‘undistinguished attainments’ and secured a post as a bookfetcher, or ‘library boy’ in the Cambridge University Library, where he developed a passion for Oriental languages. He was awarded a scholarship at St John's College, where he graduated in 1935 with a degree in Oriental Languages (Arabic and Aramaic), and then moved to Pembroke College with an E. G. Browne Scholarship to read Arabic and Persian (1936–37); the scholarship was extended to the end of 1937 at the request of R. A. Nicholson and Reuben Levy, with whom Pearson was reading Persian and ‘working reasonably hard and consistently’; he was also reading Sanskrit with Professor Bailey, ‘who tells me his work is satisfactory there’. Levy considered Pearson ‘quite worthy’ of the extension.

Pearson then returned to work in the Oriental section of the library until 1941, when he was called up for military service as a signaller; he was later transferred to the Field Security section of the Intelligence Corps and served in Germany at the end of the war. Demobilised on the last day of 1945, he returned to Cambridge and worked again in the University Library as an Assistant Under-Librarian until 1950.

In 1950 he was appointed Librarian of the School of Oriental and African Studies (SOAS) in London, and from then until 1972 oversaw the drastic expansion and development of the library in what were probably the most important years of its history. In the mid-1950s, he decided that, in addition to maintaining the normal catalogue of books, it would be useful also to compile a catalogue of the articles contained in the library's periodicals and other collective volumes. If left uncatalogued, much of this research, especially in the less obvious sources, would tend to be overlooked. With assistants at SOAS he eventually compiled a register of more than 25,000 articles published between 1906 and 1955; once classified together with the holdings of libraries other than SOAS, this valuable reference work was published as Index Islamicus.