[1][2][3][4] In 1922, she took a job at the Marine Biological Station in Millport on the Isle of Cumbrae where she worked for the rest of her life.

[5][6] From 1928 to 1929 Marshall travelled with Frederick Stratten Russell and J. S. Colman on the Great Barrier Reef Expedition led by Maurice Yonge.

[5] She retired as Deputy Director of the Station in 1964 (having been appointed to this post on the death of Orr, the previous post-holder, in 1962).

[6] Between 1970 and 1971 she attended the Scripps Institution of Oceanography in the United States and she visited the Villefranche-sur-Mer Marine Station in France 1974.

Initially Marshall was educated by governesses, later attending Rothesay Academy and St Margaret's School in Polmont.

[5][6] She held a Carnegie Fellowship at the University from 1920 to 1922 and worked with the professor of zoology, John Graham Kerr.

[11] In 1949 Marshall, along with Ethel Dobbie Currie, became the first women to be elected Fellows of the Royal Society of Edinburgh.