In 1905, she published her first novel in the Deutsche Roman-Zeitung. However, she then started to work as an actress, beginning in 1906 in Düsseldorf, then moving to Weimar (1908), Chemnitz (1911) and Aachen (1913). In Aachen she met her first husband, the actor and director Rudolf Klein-Rogge, whom she married in 1914.

In 1920, she wrote her first film script Das indische Grabmal (The Indian Tomb, Mysteries of India), together with Fritz Lang. Fritz Lang became her second husband in 1922, and they collaborated in the following years, writing the screenplays for Metropolis and M together. They separated in October 1931 and divorced in 1933.

In 1932, a year before Adolf Hitler came to power, she joined the Nazi Party. This presumably led to the divorce from Lang, who left Germany in 1934 for Paris after his film Das Testament des Dr. Mabuse had been declared illegal by Nazi officials because of perceived criticism of Nazi ideology.

Harbou wrote the script for Der Herrscher 1937, directed by Veit Harlan and starring Emil Jannings. The movie celebrates unconditional submission under absolute authority, eventually finding reward in total victory.

After the war she was detained by the British military government, and then did unskilled labor, like cleaning up rubble from the bombing. After receiving a working permit she did some synchronizing of movies, but also continued to write scripts.

In 1954 she died in Berlin.