D'Harnoncourt initially eked out a minimal living as a commercial artist, but quickly acquired a reputation for his knowledgeable advice to American antique collectors.
In 1927, d’Harnoncourt went to work for Frederick W. Davis, who operated one of the most important antiquities and folk art shops in Mexico City.
Davis was among the first to collect, display and sell the work of the emerging Mexican artists such as Diego Rivera, José Clemente Orozco, and Rufino Tamayo; others who frequented the shop included Miguel Covarrubias and Jean Charlot.
D'Harnoncourt assisted in buying and selling antiques and contemporary works and also organized displays and exhibits in the showroom.
Among many others, Margaret Lefranc was a guest speaker several times during 1934–1935, educating listeners on American and European art.