Kapteyn studied at the University of Utrecht 1868-1875 mathematics and physics and then worked at the Observatory of Leiden. Then he became the first Professor of Astronomy and Theoretical Mechanics at the University of Groningen from 1878 to 1921.
Kapteyn had no own observatory available, but specialized in the analysis of photographic plates and a comprehensive screening, especially of the Southern Sky, taken at the Observatory of Cape Town. He cataloged here 454 875 stars. His works from 1896 to 1900, exploring issues like the spectral class and the proper motion of the stars and helped considerably with to understand the current structure of the Milky Way. Kapteyn discovered in 1897 on the photographic plates by David Gill named after him Kapteyn's star. In 1904, he found out that the star movement by no means is random, but can be split into two main currents of different directions. Later he realized that he had thus found the predicted by Bertil Lindblad and Jan Oort galactic rotation. In 1922, he published his life's work, in which he developed a cosmic model in which a disc-shaped, rotating galaxy 40,000 light years in diameter, where the sun is about 2,000 light-years from the center. The approach is our current understanding of the Milky Way quite similar, but includes error due to the lack of knowledge about interstellar absorption in this time.
According to him, discovered by Max Wolf in Heidelberg on February 21, 1919 Asteroid ( 818) Kapteynia is named, also carries the Astronomical Institute of the University of Groningen and the lunar craters Kapteyn his name.
He is the brother of Willem Kapteyn.