Leo Kanner (/ˈkænər/; born Chaskel Leib Kanner; June 13, 1894 – April 3, 1981) was an Austrian-American psychiatrist, physician, and social activist best known for his work related to infantile autism. Before working at the Henry Phipps Psychiatric Clinic at the Johns Hopkins Hospital, Kanner practiced as a physician in Germany and South Dakota. In 1943, Kanner published his landmark paper Autistic Disturbances of Affective Contact, describing 11 children who displayed "a powerful desire for aloneness" and "an obsessive insistence on persistent sameness." He named their condition "early infantile autism". Kanner was in charge of developing the first child psychiatry clinic in the United States and later served as the Chief of Child Psychiatry at the Johns Hopkins Hospital. He is one of the co-founders of The Children's Guild, a nonprofit organization serving children, families and child-serving organizations throughout Maryland and Washington, D.C., and dedicated to "Transforming how America Cares for and Educates its Children and Youth." He is widely considered one of the most influential American psychiatrists of the 20th century.

Leo Kanner was born as Chaskel Leib Kanner in Klekotów, Austro-Hungarian Monarchy (present day Klekotiv, Ukraine) on June 13, 1894, to Abraham Kanner and Clara Reisfeld Kanner. In this area, approximately 70% of the total population was of Jewish descent. Kanner despised his given names, "Chaskel", which is the Yiddish version of "Ezekiel", and "Leib", instead choosing to go by "Leo". Growing up in a traditional Jewish household, Kanner received both a religious and a secular education. Kanner spent the first years of his life in Klekotów with his family and was brought up according to Jewish tradition and custom.

In 1906, Kanner was sent to Berlin to live with his uncle. Later, the rest of his family followed. At a young age, Kanner appreciated the arts and wanted to pursue a career as a poet; unfortunately, he was not able to get his works published. In 1913, Kanner graduated from the Sophien-Gymnasium, a public state high school in Berlin, where he excelled in the sciences. He then passed the graduating Staatsexamen exam in 1919 and enrolled at the University of Berlin medical school. However, Kanner's medical education was interrupted during World War I, when he was recruited to serve in the Austro-Hungarian Army in the medical service of the 10th Infantry Regiment. After the war, Kanner went back to medical school in Berlin and officially received his medical degree in 1921. Later that year, Kanner married June Lewin, with whom he would eventually have two children: Anita (born in 1923) and Albert (born in 1931).

After graduating medical school, Kanner worked as a cardiologist at the Charité Hospital in Berlin. Kanner began doing work with normal heart sound to the relationship of the electrocardiogram. At that time, the atmosphere at the Charité clinics and institutes inspired rapid progress in science, teaching and patient care. The Charité, situated in the middle of Berlin, attracted students, physicians and scientists from all over the world, resulting in a group of outstanding personalities and renowned clinicians.

Motivated by the post-war hyperinflation and poor economic conditions of Weimar Germany, Kanner immigrated to the United States in 1924. If he had stayed in Germany his fate might have been similar to other Jewish professionals who lost their lives during the war. He stated: "Little did I know, if I had remained in Germany I would have been perished by Hitler in the Holocaust".^{[failed verification]}

When he emigrated to the United States in 1924, he worked at the state hospital in Yankton, South Dakota, where he started his pediatric and psychiatric studies. Upon arrival, Kanner was appointed assistant physician at the Yankton State Hospital. It was there Kanner would learn the subtleties of pediatrics and psychiatry, two fields in which he was not experienced. To enhance his command of the English language, Kanner did the crossword puzzles in The New York Times. During his time in South Dakota, Kanner published his first works, which were on general paralysis and syphilis. Kanner also studied the effects of adrenalin on the blood pressure of patients with functional paralysis. Additionally, he published his first book, Folklore of the Teeth, an analysis of dental practices around the world in relation to customs and folklore, in 1928.

After serving four years in South Dakota, Kanner attained a fellowship position at the Henry Phipps Psychiatric Clinic at the Johns Hopkins Hospital in Baltimore, Maryland in 1928, after attracting the attention of Adolf Meyer who was the Psychiatrist-in-Chief and Director of the psychiatric clinic. In 1930, with monetary support from the Macy and Rockefeller Foundations, Meyer and Edward A. Park were able to establish the Children's Psychiatry Service at the Harriet Lane Home at Johns Hopkins, which was the first child psychiatry clinic in the United States, and appointed Kanner to develop the program. Despite his inexperience in the fields of pediatrics and child psychiatry, Kanner was able to teach himself pediatric psychiatry. In 1933, Meyer promoted Kanner to associate professor of psychiatry at Johns Hopkins University. In 1935 the first edition of his textbook, Child Psychiatry was published. This was the first English language textbook on child psychiatry.

Beyond his revolutionary clinical research, Kanner's concern for the mentally ill manifested as social activism for which he is also remembered today. In the 1930s, a group of lawyers and judges arranged for 166 state-institutionalized, mentally ill residents to be released and assigned as unpaid domestic servants for affluent families around Baltimore. The release of these patients was justified through the Habeas Corpus writs and the claim to familial rights. Out of his own concern, Kanner decided to track down the 166 patients and found them plagued with a variety of dreadful outcomes such as STDs, tuberculosis, prostitution, imprisonment, institutionalization, and even death. Kanner reported that the 166 released patients had a total of 165 children, many of whom became orphans or died due to neglect. Kanner's report on these patients, "Scheme to Get Morons to Work in Homes Free Charged", made the headlines of The Baltimore Sun and The Washington Post in 1938. The publicity helped spark community action and led to the better treatment of the mentally ill. Apart from his social activism for the mentally ill, during the run-up to World War II, Kanner was instrumental in rescuing hundreds of Jewish physicians from the horrors of the Nazis by relocating them to work in the United States. He and his wife opened their home to many of these European refugees.