I will present first results from a galaxy population study in the five most distant (z~1.4-1.8) clusters identified in the South Pole Telescope Sunyaev Zel’dovich survey (SPT-SZ). With a dedicated 4-band HST and Spitzer follow-up of the complete z>1.4 tail of the SPT-SZ cluster sample, we investigate galaxy populations in these rare structures, among the few very massive clusters known at this redshift which is deemed to be a transformational time for massive cluster galaxies, transitioning from their main formation epoch at z>~2 to the passive evolution regime at z<1. Based on photometric classification of quiescent and star-forming galaxies, we measure quiescent galaxy fractions implying environmental quenching efficiencies in the range ~0.5-1 in the inner r<0.5xr500 region. Although direct comparison with other studies at similar redshifts is complicated by differences in cluster masses, probed cluster regions, and observations, this work suggests that at least in the central regions of the most massive structures the suppression of star formation in cluster galaxies with respect to their field analogs already efficiently occurs earlier than z~1.4.