Since the beginning of its operation in April 2005, the MAGIC telescope was able to observe ten different GRB events since their early beginning, even while the prompt emission was still ongoing. Observations, with energy thresholds spanning between 80 and 300 GeV, did not reveal any γ-ray emission. We present a direct determination of the MAGIC sensitivity in GRB mode and the upper limits for the ten follow-up observations. At energies around 100 GeV, MAGIC is currently the fastest and most sensitive operational GRB detector in the world.