He wrote many books including The Human Mind (1930), Man Against Himself (1938), Love Against Hate (1942), The Vital Balance (1963) and The Crime of Punishment (1968).

During his life he advocated for a number of causes including children suffering from abuse or neglect, Native Americans, women's rights, prisoners, the elderly, the environment, wildlife, and against nuclear weapons.

After his internship and his service as part of the Naval Reserve during World War I, Menninger worked at the Boston Psychopathic Hospital under Elmer Ernest Southard while also teaching neuropathology at Harvard Medical School.

After World War II, Karl Menninger was instrumental in founding the Winter Veterans Administration Hospital, in Topeka.

[7] While the clinic housed Freudian analysts, there was no commitment to any one form of therapy and a belief in the therapeutic value of a warm and caring environment.

In 1930, he wrote his first book The Human Mind, where he argued that psychiatry was a science and that the mentally ill were only slightly different from healthy individuals.

[2] He also, despite calling himself "more Freudian than Freud" was not particularly attached to doctrine from that tradition, referring to organized psychoanalysis as "the Vatican".

They were both interested in pediatric psychiatry[4] In 1988, Karl Menninger wrote a letter to Thomas Szasz, a psychiatrist who viewed mental illness to be a myth.

After recounting the lack of scientific method in psychology over the years, Menninger expressed his regret that he did not come over to a dialogue with Szasz.

[14] He also called homosexuality an evil and a sin in a 1963 introduction to the American edition of the Wolfenden Report [15] He was among the first members of the Society for General Systems Research.