Mini-EUSO is a wide Field-of-View (44∘×44∘) telescope currently in operation from a nadir-facing Ultra-Violet (UV) transparent window in the Russian Zvezda module on the International Space Station (ISS). Mini-EUSO has been designed as a scaled-down version of the original JEM-EUSO telescope to raise its instrumentation's technological readiness level and demonstrate its observational principle, while performing multidisciplinary studies on different fields such as atmospheric science and planetology. One of Mini-EUSO main goals is the study of the UV background for future space missions employing the same concept as the original JEM-EUSO telescope, which requires an absolute calibration of the Mini-EUSO instrument. During the past years, a few observational campaigns have been completed, employing a ground-based UV flasher to perform an end-to-end calibration of the instrument. In this paper, we present the assembled UV flasher system, the operation of the field campaign and the analysis of the obtained data. The results are interpreted by the means of a parametrization of the Mini-EUSO photon counts. The end-to-end efficiency of several pixels has been obtained, taking into account different parameters such as the atmospheric attenuation, the optics efficiency and the multi-anode photomultiplier detection efficiency.