A native of Brooklyn, Duncan was educated at the Northfield Seminary in Northfield, Massachusetts, after her family relocated there. In 1896 she took a work/study position at Long Island’s prestigious Parsons Nursery, where she studied the propagation and culture of woody ornamentals under Samuel Parsons, Sr. and Jean Rudolf Trumpy. Four years later, Duncan began a career as a garden writer. Between 1901 and 1926 she wrote articles for numerous magazines, including Atlantic Monthly, the Century, and Scribner’s. Duncan wrote the first gardening column for the Ladies’ Home Journal, and frequently contributed to Country Life in America and The Garden Magazine. She covered such topics as horticulture, design issues, descriptions of historic gardens in the Charleston, South Carolina, area, and the gardens of artists and writers in the Cornish, New Hampshire, art colony. Duncan also published several books, including two garden books for children entitled Mary’s Garden and How it Grew (1904) and When Mother Lets Us Garden (1909); two garden novels, My Garden Doctor (1914) and Roberta of Roseberry Gardens (1916); and two gardening books, The Joyous Art of Gardening (1917) and Home Vegetables and Small Fruits (1918).
Duncan was one of the first women to pursue a full-time, professional career as a garden writer. She operated the Gardencraft Toy Company for several years, and was involved with the school gardens movement at Montessori and Ethical Culture schools in New York and California. Prior to moving to California and becoming a garden editor at the Los Angeles Times in 1925, Duncan divided her time between New York City and Cornish. In 1930 a nearly-fatal automobile accident left her unable to write for several years. She later continued her work with school gardens in the Los Angeles area. Duncan died at the age of 95 and was buried in Live Oak Memorial Park in Monrovia, California.