Klaus moved to the USA to escape Nazism, and after training in counterintelligence as one of the Ritchie Boys, he served in Europe during the World War II, becoming one of the first outsiders to witness the horrors of the concentration camps.

His books Escape to Life (co-written with his sister Erika Mann), and The Turning Point have attained a historical importance as frequently cited primary documents of the experience of exile undergone by members of the German intelligentsia and arts community who fled the Third Reich.

He is best known for his 1936 novel, Mephisto, about an actor who sells his soul to the devil, by attaching his career to the rise of the Nazis, which was made into a film of the same name, in 1981—a book that was banned in Western Germany after the war.

[5] Mann's early life was described by him as romantic, in beautiful upper-class surroundings (Je suis de mon temps.

During his early travels often by car throughout Europe and North Africa as well as America and Asia, Erika took the wheel and determined the itinerary to compensate for Mann's inability to make personal and useful decisions.

[7] During the time Erika travelled with Klaus to North Africa in 1929, they met Annemarie Schwarzenbach, a Swiss writer and photographer, who remained close to them for the next few years.

Initially, the aspiring writer used opium, Eukodal and later heroin, possibly to increase his creative energy, as artists and intellectuals in literary circles often did at the time.

It was primarily affiliated with a number of influential German writers who fled from the Hitler regime during the first years of the establishment and consolidation of Nazi rule, but other internationally acclaimed authors such as Aldous Huxley and Heinrich Mann contributed essays and editorial work.

After fleeing to Paris on receiving news of the Nazis' rise to power, worried about his communist past, he is helped by a former co-actress from Hamburg, Angelika Siebert, who travels to Berlin to convince the girlfriend of a Luftwaffe general to have him pardoned.

On obtaining the role of Mephisto in Faust Part One he realizes that he actually made a pact with evil (i.e. Nazism) and lost his humane values (even denouncing his mistress as "Black Venus").

After seven years of legal hearings, the West German Supreme Court upheld the ban, although it continued to be available in East Germany and abroad.

In the summer of 1937, he met his partner for the rest of the year, Thomas Quinn Curtiss, who was later a longtime film and theater reviewer for Variety and the International Herald Tribune.

Mann's novel Der Vulkan was published in 1939 at the onset of World War 2, and was another attempt by him to portray not only Germans but other European exiles during that time.

The process of naturalization was delayed because of an investigation the FBI conducted into Klaus Mann's political and sexual activities, as he was openly gay, but not an adherent of marxist ideologies.

Klaus was engaged in psychological warfare designing leaflets intended for German soldiers stationed in Italy and North Africa.

In his second 1942 autobiography (Der Wendepunkt, the Turning Point), he critically observed the segregation of white army personnel from their black counterparts at Camp Ritchie.

As he visited liberated concentration camps in official function, he was one of the first eye-witnesses to report on the horrors of mass extermination during Nazi rule in Germany.