Stark worked in various positions at the Physics Institute of his alma mater until 1900, when he became an unsalaried lecturer at the University of Göttingen. An extraordinary professor at Hannover by 1906, in 1908 he became professor at RWTH Aachen University. He worked and researched at physics departments of several universities, including the University of Greifswald, until 1922. In 1919, he won the Nobel Prize in Physics for his "discovery of the Doppler effect in canal rays and the splitting of spectral lines in electric fields" (the latter is known as the Stark effect). From 1933 until his retirement in 1939, Stark was elected President of the Physikalisch-Technische Reichsanstalt, while also President of the Deutsche Forschungsgemeinschaft.
It was Stark who, as the editor of the Jahrbuch der Radioaktivität und Elektronik, asked in 1907, then still rather unknown, Albert Einstein to write a review article on the principle of relativity. Stark seemed impressed by relativity and Einsteins earlier work when he quoted "the principle of relativity formulated by H. A. Lorentz and A. Einstein" and "Plancks relationship M = E/c" in his 1907 paper in Physikalische Zeitschrift, where he used the equation e = mc to calculate an "elementary quantum of energy", i.e. the amount of energy related to the mass of an electron at rest. While working on his article, Einstein began a line of thought that would eventually lead to his general theory of relativity, which in turn became (after its confirmation) the start of Einsteins worldwide fame. This is ironic, given Starks later work as an anti-Einstein and anti-relativity propagandist in the Deutsche Physik movement.
Stark published more than 300 papers, mainly regarding electricity and other such topics. He received various awards, including the Nobel Prize, the Baumgartner Prize of the Vienna Academy of Sciences (1910), the Vahlbruch Prize of the Göttingen Academy of Sciences (1914), and the Matteucci Medal of the Rome Academy. Probably his best known contribution to the field of physics is the Stark effect, which he discovered in 1913.  In 1970 the International Astronomical Union honored him with a crater on the far-side of the Moon, without knowing about his Nazi activities. The name was dropped on August 12, 2020.
He married Luise Uepler, and they had five children.  His hobbies were the cultivation of fruit trees and forestry. He worked in his private laboratory, which he set up using his Nobel prize money, on his country estate in Upper Bavaria after the second world war.  There he studied the deflection of light in an electric field.