Sheina Marshall was born on the Isle of Bute and educated at the University of Glasgow, where she graduated with a BSc in zoology in 1919. She held a Carnegie Fellowship from 1920 to 1922 and was awarded a doctorate in science in 1934.
From 1922 she worked at the Marine Biological Station at Millport on the Isle of Cumbrae in the Firth of Clyde, where she remained for her entire career. Marshall’s work, alongside Andrew Pickin Orr, led to an invitation to join an expedition to the Great Barrier Reef, off the Australian coast, and with Orr she published The Biology of a Marine Copepod (1957).
During the Second World War Marshall took part in the search of British coasts for seaweeds suitable as a source of agar. She retired as deputy director of Millport in 1964 but remained there as an Honorary Fellow, after the universities of London and Glasgow had taken the station's management in 1970. It was in 1963 that Marshall became a Fellow of the Royal Society and she was made an OBE three years later.