Understanding the rapid formation of supermassive black holes (SMBHs) in theearly universe requires an understanding of how stellar mass grows in the hostgalaxies. Here, we perform an analysis of rest-frame optical spectra andimaging from JWST of two quasar host galaxies at z>6 which exhibit Balmerabsorption lines. These features in the stellar continuum indicate a lack ofyoung stars, similar to low-redshift post-starburst galaxies whose starformation was recently quenched. We find that the stellar mass (log(M_* /M_sun) > 10.6) of each quasar host grew in a starburst episode at redshift 7 or8. One of the targets exhibits little ongoing star formation, as evidenced bythe photometric signature of the Balmer break and a lack of spatially resolvedH-alpha emission, placing it well below the star formation main sequence at z =6. The other galaxy is transitioning to a quiescent phase; together, the twogalaxies represent the most distant massive post-starburst galaxies known. Thematurity of these two galaxies is further supported by the stellar velocitydispersions of their host galaxies, placing them slightly above the upper endof the local M_BH - sigma_* relation. The properties of our two post-starburstgalaxies, each hosting an active SMBH with log(M_BH / M_sun) > 9, suggests thatblack holes played a major role in shaping the formation of the first massivegalaxies in the Universe.