By the time of her death in 2003, she had become almost the last direct link with two generations of Irish and British Victorian conchologists who brought distinction to the subject through their personal research and field collecting.
Largely self-taught, she was conferred an Honorary MSc by the University of Liverpool in 1991, was President of the Conchological Society of Great Britain and Ireland, and was elected a Member of the Royal Irish Academy in 1970.
However her interest in shells had been sparked by summer visits to the beach at Millisle at the age of six years and, encouraged by a family friend, H.C. Lawlor, who introduced her to the photographer and malacologist Robert Welch, she joined the Belfast Naturalists' Field Club.
During the 1920s her growing expertise in conchology was nurtured by Welch and other prominent members of the Field Club such as Robert Lloyd Praeger, the geologist John Kaye Charlesworth, and especially Arthur Stelfox, who was a major influence on her.
In 1973, by now a respected curator, researcher and author at the Liverpool Museum, she reduced to part-time and finally retired in 2000 at the age of 92, though she continued to be in demand for her expertise.
[5] McMillan herself travelled widely in Europe, to Lake Chad in Africa, Australia and New Zealand, and went alone on a shell collecting expedition to a whaling station in the Arctic Ocean in the early 1970s.
Known by many as 'Mrs Mac,' she lived in some chaos, surrounded by heaps of papers and books on every floor, table and any other surface, and climbed over by dozens of 'rescued' stray cats.
[2] Much of her personal life was an extension of her academic interests, and she wrote prolifically on many subjects, including local history; she was a member of the Bromborough Society.