He was born at Gießen, the son of Joseph Hillebrand, a literary historian and writer on philosophic subjects.

Karl Hillebrand became involved, as a student in Heidelberg, in the Baden revolutionary movement, and was imprisoned in Rastatt in 1849. He succeeded in escaping and lived for a time in Strassburg, Paris ... where for several months he was Heinrich Heine's secretary ... and Bordeaux. He continued his studies, and after obtaining the doctor's degree at the Sorbonne, he was appointed teacher of German in the Ecole militaire at St Cyr, and shortly afterwards, professor of foreign literatures at Douai.

On the outbreak of the Franco-German War, he resigned his professorship and acted for a time as correspondent to The Times in Italy. He then settled in Florence, where he died on the 19th of October 1884.

Hillebrand wrote with facility and elegance in French, English and Italian, besides his own language. His essays, collected under the title Zeiten, Völker and Menschen (Berlin, 1874-1885), show clear discernment, a finely balanced cosmopolitan judgment and grace of style.

He undertook to write the Geschichte Frankreichs von der Thronbesteigung Ludwig Philipps bis zum Fall Napoleons III, but only two volumes were completed (to 1848) (2nd ed., 1881-1882). In French he published Des conditions de la bonne comedie (1863), La Prusse contemporaine (1867), Etudes italiennes (1868), and a translation of O. Müller's Griechische Literaturgeschichte (3rd ed., 1883).

In English, he published Public Instruction in the United States (1869) and his Royal Institution Lectures on German Thought during the Last Two Hundred Years (1880). He also edited a collection of essays dealing with Italy, under the title Italia (4 vols, Leipzig, 1874-1877).