Klara Johanson was born in Halmstad as the daughter of the milliner and furrier Alexander Johanson and his wife Anna Christina. In 1894 she became the first woman in Halmstad to present herself for the upper secondary school leaving examination, and she graduated from university in 1897. In her university days, she was an active member of the Kvinnliga Studentföreningen (the women's student association) in Uppsala. From 1898 to 1901, she worked as a sub-editor for the women's periodical Dagny and until 1912 was a critic for the newspaper Stockholms Dagblad, for which she also wrote short, humorous pieces under the pen name Huck Leber; she was described as Sweden's most eloquent critic with the highest aesthetic sensitivity.
Together with her lifelong friend Ellen Kleman, she published Fredrika Bremers brev in four volumes from 1915 to 1920, followed by the reflective book En recensents baktankar, 1928, and the collection of literary essays Det speglade livet, 1926, and Det rika stärbhuset, 1946, in which she displayed her knowledge of Goethe, Emerson, Kierkegaard, Bremer, and Amiel, among others. Her writing also includes the combative publications Den undre världen, 1907, a prostitute's diary, and Sigrid Fridman – och andra konstnärer, 1948, which defends the female artist in a number of topical art disputes.
She translated works such as H. F. Amiel's Fragments d'un journal intime (1883; Eng. tr. Amiel's Journal: The Journal Intime of Henri-Frederic Amiel), and Rosa Mayreder's Geschlecht und Kultur, 1923. She also wrote four student plays and articles for, for example, the papers Tidevarvet and Ord & Bild. In 1952, N. Afzelius published the book K. J. själv, which includes childhood memories, in 1953 the selection of letters in Brev, and in 1957 the selection Kritik. In 1989 her work Ur dagböckerna was published in a selection by G. Eman.