Klara Johanson was born in Halmstad, Sweden, the daughter of a milliner and his wife. In 1894, she was the first woman in her hometown to pass the upper secondary school final examinations. She then attended Uppsala University, where she received a master's degree in the humanities in 1897. As a student, she became known as a witty writer for student publications. After graduating, she moved to Stockholm, where she became an editor of Dagny, the magazine of the Fredrika Bremer Association, the first Swedish women's rights organization. She also wrote for the newspaper Stockholms Dagblad, contributing literary criticism under her initials KJ and poetry and humorous stories under the pen name Huck Leber (inspired by Huckleberry Finn). Beginning in 1899, she also translated works by Knut Hamsun, Abbé Prévost, Benedetto Croce, Rosa Mayreder, Henri-Frédéric Amiel, and others, introducing them to the Swedish public. Under her pseudonym Huck Leber, she published a collection of her own stories called Oskuld och arsenik (Virgin and Arsenic) in 1901. She edited Den undre världen (1907), a diary written by a prostitute that proved to be highly controversial. In 1912, she began living with Ellen Kleman, the woman who would be her life's companion and colleague. Between 1915 and 1920, they jointly published a selection of Fredrika Bremer's letters in four volumes with a comprehensive commentary. In the 1920s and early 1930s, Johanson wrote for the radical and pacifist weekly magazine Tidevärvet, published by the Liberal Women's Union. She also published two collections of her essays in 1926 and 1946, mostly on Swedish and German fiction, but also on Kierkegaard and American literature. Her complete writings, including letters and a selection of criticism, were published in five volumes in 1947-1957.