The goal of the Karlsruhe Tritium Neutrino (KATRIN) experiment is to measure the effective electron anti-neutrino mass with a sensitivity of 200 meV/c² (90 % C.L.), by investigating the energy spectrum of tritium beta-decay electrons close to the endpoint. The first neutrino mass measurement campaign took place in spring 2019. With this first dataset an upper limit on the neutrino mass of 1.1 eV (90 % C.L.) was obtained, improving upon previous limits by nearly a factor of two. In this talk, the operating principle and the first result of the KATRIN experiment are presented. A special focus will be on background investigations and their relevance in view of this first measurement.