Wiesel was born in Uppsala, Sweden in 1924, the youngest of five children. In 1947, he began his scientific career in Carl Gustaf Bernhard's laboratory at the Karolinska Institute, where he received his medical degree in 1954. He went on to teach in the Institute's department of physiology and worked in the child psychiatry unit of the Karolinska Hospital. In 1955 he moved to the United States to work at Johns Hopkins University School of Medicine under Stephen Kuffler. Wiesel began a fellowship in ophthalmology, and in 1958 he became an assistant professor. That same year, he met David Hubel, beginning a collaboration that would last over twenty years. In 1959 Wiesel and Hubel moved to Harvard University. He became an instructor in pharmacology at Harvard Medical School, beginning a 24-year career with the university. He became professor in the new department of neurobiology in 1968 and its chair in 1971.