Born in Westphalia, Germany, he joined the Society of the Divine Word (SVD) in 1922, after attending schools run by the order. Completing his basic philosophical-theological studies in 1929, he was ordained a priest. In the following years (1929-32), he was a teacher at the SVD school in St. Wendel (Saarland). Then, he studied ethnology at the University of Vienna for the newly established Anthropos Institute. He graduated in 1935 and was appointed the editor of the journal Anthropos. However, the SVD superior general sent him to China in summer 1936, with a task to take up one of the leadership positions at the newly founded (1925) Catholic University Fu-Jen in Beijing. Rahmann took up the challenge and helped to lead the university through a politically very difficult time. In 1937, the Japanese took Beijing and tightened the grip also on the Chinese educational institutions. Despite the difficulties, the university kept growing. Rahmann was also the editor (1936-46) of a new scholarly journal Monumenta Serica and co-founder of an ethnographical museum at the university.
He left China in 1949 and spent some time at the house of Anthropos Institute near Fribourg in Switzerland. In 1952, he was appointed a member of the teaching staff of San Carlos University in Cebu City, Philippines and became the dean of the Graduate School.
He saw his move to a new place as a chance to conduct ethnological fieldwork among the Negrito groups in northeast Mindanao and made his first research trip together with his Filipino colleagues already in 1953. The field research among the small-in-stature ethnic groups was at that time still an important task. He conducted that research in the following years and published the results.
In 1955, the SVD superior general called Rahmann back to Europe to be the editor of Anthropos journal. Rahmann took up again a challenge of running an institution (the journal and the Institute) through a very turbulent time. Wilhelm Schmidt had died in 1954 and Fritz Bornemann resigned as the director of the Institute in 1955 but the situation at the headquarters of the Institute was still marred by the earlier strong disagreements between the two. During his time as the journal’s editor (1955-58), Rahmann diligently saw to it that the journal continued to be published. Meanwhile, he was appointed professor of ethnology at the Fribourg University (1956).
In 1960, he returned to the Philippines and was appointed professor of ethnology at San Carlos University. During his time as the president of that university (1964-70), he built up a wide network of international contacts and strengthened the position of the university. He founded a museum (consisting of four sections: Spanish colonial period; ethnography; archaeology; natural sciences). He also founded a book series “San Carlos Publications” (1964) and was active in establishing a journal Philippine Quarterly of Culture and Society (1973). He continued his research among the Negritos and his publication activities.