Mary Nicol Neill Armour was born near Hamilton in Lanarkshire, Scotland. After attending the Hamilton Academy, where her art teacher was Penelope Beaton, who later went on to teach at the Edinburgh College of Art, she enrolled at The Glasgow School of Art. She attended the school from 1920 until 1925, where she trained under Maurice Greiffenhagen and David Forrester Wilson. Afterwards, she worked as an art teacher and, in 1927, married the painter William Armour. From 1951 she taught still life painting at the Glasgow School of Art until she retired in 1962.

Armour exhibited regularly at the Royal Academy, the Royal Scottish Academy, the Royal Glasgow Institute of the Fine Arts, the Royal Society of Painters in Watercolours and the Scottish Society of Artists. She was elected an associate of the Royal Scottish Watercolour Society in 1941, became a full member in 1956 and, two years later, became a Royal Scottish Academician, the second woman to do so. The Royal Glasgow Institute of Fine Arts awarded her the Cargill Prize in 1972. She became a member of the institute five years later and Honorary President in 1983. She was also awarded an Honorary LLD from the University of Glasgow.

Armour’s work is represented in various collections such as the Glasgow Municipal Gallery, the Edinburgh Corporation and the Art Galleries of Aberdeen.