Fokker studied mining engineering at the Delft University of Technology and physics at the University of Leiden with Hendrik Lorentz, where he earned his doctorate in 1913. He continued his studies with Albert Einstein, Ernest Rutherford and William Bragg. In his 1913 thesis, he derived the Fokker–Planck equation along with Max Planck. After his military service during World War I he returned to Leiden as Lorentz' and Ehrenfest's assistant. Fokker became a physics teacher at the Gymnasium of Delft after 1918 and was appointed in 1923 as the first professor of Applied Physics at the Technische Hoogeschool Delft (today Delft University of Technology).