In 1955, Jewett started working as staff historian at the newly-established Georgia Historical Commission (GHC), where her father C. E. Gregory, who had helped to establish the agency, was in change.
[1] Under Jewett's 13-year leadership, the GHC expanded to employ fifty people and was nationally recognized for its pioneering preservation work.
It acquired and restored twenty sites across the state (staffing a total of 15 locations and developing seven museums), and installed 1,800 historical markers.
Its full renovations unfolded over years, and the GHC ordered a study of the site by UGA archaeologist Clemens de Baillou ("The Chief Vann House at Spring Place, Georgia", published in 1957 in Early Georgia) and a translation of Moravian missionary diaries in an effort to, in Jewett's words, "[explore] every known source of knowledge and [make] the house and grounds as clear a picture as possible of the life of the owners".
Today, the Georgia Trust annually bestows the Mary Gregory Jewett Award for "distinguished service in the field of preservation" in the state.