She is known for her archaeological expeditions to Tres Zapotes and other sites in Southern Mexico in the 1940s, conducted alongside her husband Matthew Stirling, which according to National Geographic "essentially rewrote Mesoamerican history".

[1] Needing to look up the word "ethnology" in a dictionary before she started the job, she studied anthropology under Truman Michelson at George Washington University to better understand the field.

[5] During this time she trained in field archaeology alongside a number of young scholars who would go on to become prominent figures, including Gordon Willey, James A. Ford, Jesse D. Jennings, and Marshall T.

While Marion, pregnant with her first child, visited Mitla and Monte Albán, Matthew traveled eight hours on horseback from Tlacotalpan to Tres Zapotes, to see the Olmec colossal head discovered there by José María Melgar y Serrano in 1862.

He found that the sculpture was surrounded by a substantial archaeological site and, upon returning to the United States, the Stirlings obtained grants from the National Geographic Society and Smithsonian Institution to explore the area further.