Frenssen was born in the village of Barlt, in the Duchy of Holstein. He was educated at the universities of Tübingen, Berlin and Kiel. He took orders and from 1892 to 1902 was pastor at Hemme, taking his degree as doctor of theology at Heidelberg in 1903. But he had already for some years been known as a writer of novels, and in 1902, a year after his great success with Jörn Uhl (1901), he gave up his pastorate and devoted all his time to literature.
In his later years he abandoned Christianity because Christian morals were in conflict with his racialism. Instead, he turned to a form of Germanic Neopaganism which also suited his liberal views on sexuality.