We study the stellar mass functions (SMFs) of star-forming and quiescent galaxies in 11 galaxy clusters at 1.0<z<1.4, drawn from the Gemini Observations of Galaxies in Rich Early Environments (GOGREEN) survey. Based on more than 500 hours of Gemini/GMOS spectroscopy, and deep multi-band photometry taken with a range of observatories, we probe the SMFs down to a stellar mass limit of 10^9.7 Msun (10^9.5 Msun for star-forming galaxies). At this early epoch, the fraction of quiescent galaxies is already highly elevated in the clusters compared to the field at the same redshift. The quenched fraction excess (QFE) represents the fraction of galaxies that would be star-forming in the field, but are quenched due to their environment. The QFE is strongly mass dependent, and increases from ~30% at Mstar=10^9.7 Msun, to ~80% at Mstar=10^11.0 Msun. Nonetheless, the shapes of the SMFs of the two individual galaxy types, star-forming and quiescent galaxies, are identical between the clusters and the field - to high statistical precision. Yet, along with the different quiescent fractions is the total galaxy SMF environmentally dependent, with a relative deficit of low-mass galaxies in the clusters. These results are in stark contrast with findings in the local Universe, and thus require a substantially different quenching mode to operate at early times. We discuss these results in the light of several popular quenching models.
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