She authored numerous exhibition catalogs, including the seminal The First Century of Printmaking, 1400 to 1500 and the catalogue raisonné of the prints of Paul Gauguin.

[2] After graduation, she was hired by Winslow Ames to work at the small Lyman Allyn Art Museum in New London, Connecticut.

After teaching briefly in Florence, through Sachs she was interviewed and hired in 1937 by art collector Lessing J. Rosenwald to curate his private collection of some 5000 prints, to be housed at his home Alverthorpe Manor, in Jenkintown, Pennsylvania.

[2] Rosenwald later wrote of Mongan: As anyone with reasonable intelligence would, I leaned very heavily on her judgment, discernment, and ability during the most active portion of my collecting career.

[4] In 1986, Harvard University instituted the I Tatti Mongan Prize for "a distinguished scholar of Renaissance art or connoisseurship who carries into a new generation the qualities of imaginative scholarship, personal generosity, and devotion to the institutions of art history that were exemplified in their own generation by Agnes and Elizabeth Mongan.