Born in Baden, Germany, Westermann studied at Basel and Tübingen prior to being sent to Togoland (modern Togo) by the North German Mission as a teacher. There he developed an interest in the Ewe language that was to shape the direction of the rest of his life. Two major publications from this period were Wörteruch der Ewe-Sprache (vols. 1 and 2, 1905, 1906) and Grammatik der Ewe-Sprache (1907). In 1908 he left missionary service to begin lecturing on Ewe, Ful, and Hausa at the Orientalisches Seminar in Berlin. In 1910 he replaced the Bantuist Carl Meinhof as professor, later taking up the chair of African Languages and Cultures at Berlin University, where he remained until retiring in 1950. The studies he had begun in Togoland led to an interest in the Sudanic languages, early results of which were published as Die Sudansprachen (1911), but his classification of the languages was continually modified over the years, most notably in “Charaketer und Einteilung der Sudansprachen” (Africa, 1935) and, with M. A. Bryan, The Languages of West-Africa (1952). (“Sudan” in that period included most of West Africa as well as areas to the east.) His main interest was in the languages at the western end of the family, such as Mande or Kpelle and Gola of Liberia, but his bibliography shows extensive publications on languages across West Africa. From 1928 to 1939, along with D. G. Brackett (1929-1939), he served as editor of Africa, the journal of the International Institute of African Languages and Cultures (now the International African Institute).
Westernmann particularly enjoyed working on phonetics and tonetics. Practical Phonetics for Students of African Languages (1933, 1973), written with Ida C. Ward, benefited students for many years. Cultural change and African development also had his attention, and Geschichte Afrikas (1952) was an attempt at a comprehensive survey of the history of the African people. The cumulative effect of his work was to help Africanists see the continent as an indivisible whole.