McKelvey developed an interest in landscape design and started volunteering at the Arnold Arboretum with Charles Sprague Sargent.
Soon, her interest shifted from landscape architecture to botany and she undertook a collecting expedition to Glacier National Park, followed by further fieldwork in the White Mountains in New Hampshire.
The culmination of this research, The Lilac: A Monograph,[5] was published 1928 to great reviews, the first book to receive the Schaffer Medal of the Pennsylvania Horticultural Society.
McKelvey's work in this area culminated in two volumes entitled Yuccas of the Southwestern United States, Part one and two,[6] published nine years apart.
In 1931, she became a research associate for Oakes Ames at the Arnold Arboretum and collaborated with the Botanical Museum in Cambridge, now part of the Harvard University Herbaria.