After service as astronomer of the United States Northwest Boundary Survey, in 1873 he travelled to Europe, becoming the first American to obtain a doctorate in the laboratory of Rudolf Leuckart; receiving his Ph.D. in Zoology from the University of Leipzig in 1876.

Bringing the cytological and histological approach with him to Harvard University in 1877, he was responsible for the introduction of advanced European microscopic techniques.

After completing their doctorates, graduates of Mark’s laboratory dispersed across the United States having a profound effect on the progress of American zoology.

Mark's students accepted academic appointments at universities and scientific institutions, founded or expanded natural history museums, founded marine laboratories, and contributed lasting scientific research legacies.

[1][2] A prolific author, elected to the National Academy of Sciences in 1903, Mark was known for his insistence on orderliness, accuracy of detail, and attention to bibliographic data; setting a high standard for research and publications in America.