She married Alex Arbuthnot and worked on the family's dairy farm. In addition to her botanical activities, she was active in the community as chair of the Nambrok School Committee, chair of the Sale District School Committee and, with fellow dairy farmers, she started a Macalister Dairy Ladies Group. In the meantime, she managed the family and the 400–500 cow dairy farm. Also active in politics, in 2006, Isobel received an honorary life membership for her distinguished service to the National Party of Australia.[8]
She died in Newlands, Cape Provence, South Africa in 1963.[4]
The plant Lampranthus arbuthnotiae, a succulent, is named after her.[4]