After her undergraduate studies, Horner remained at Newnham College, becoming in 1918 an assistant librarian and then, in 1920, acting librarian. In 1921, Horner traveled to Ceylon (Sri Lanka), India and Burma where she was first introduced to Buddhism, its literature and related languages. In 1923, Horner returned to England where she accepted a Fellowship at Newnham College and became its librarian. In 1928, she became the first Sarah Smithson Research Fellow in Pali Studies. In 1930, she published her first book, Women Under Primitive Buddhism. In 1933, she edited her first volume of Pali text, the third volume of the Papancasudani (Majjhima Nikayacommentary). In 1934, Horner was awarded the title of an M.A. from Cambridge. From 1939 to 1949, she served on Cambridge's Governing Body.
In 1936, due to Butler's accepting a position at Manchester University, Horner left Newnham to live in Manchester. There, Horner completed the fourth volume of the Papancasudani (published 1937). In 1938, she published the first volume of a translation of the Vinaya Pitaka. (She was to publish a translation of the last Vinaya Pitaka volume in 1966.)
In 1942, Horner became the Honorary Secretary of the Pali Text Society (PTS). In 1943, in response to her parents' needs and greater PTS involvement, Horner moved to London where she lived until her death. In 1959, she became the Society's President and Honorary Treasurer.
In 1964, in recognition of her contributions to Pali literature, Horner was awarded an honorary Ph.D by Ceylon University.
In 1977, Horner received a second honorary Ph.D from Nava Nalanda Mahavihara.
Early Buddhist theory of man perfected : a study of the Arahan concept and of the implications of the aim to perfection in religious life, traced in early canonical and post-canonical Pali literature (1936/1975)
Book of the discipline (Vinaya-pitaka) (1938), translated by I. B. Horner