Annette Hoyt Flanders was born in Milwaukee, WI, the daughter of Frank M. Hoyt, a prominent attorney, and Hettie Pamelia Hoyt. She received her early education from tutors and at private schools. She majored in botany at Smith College, earning her B.S. in 1918, and received her M.A. in landscape architecture from the University of Illinois. She studied civil engineering at Marquette University, as well as design, architecture and the history of architecture at the Sorbonne. She served with the American Red Cross in France in 1918 and 1919. Upon her return, she joined the landscape architecture firm of Vitale, Brinckerhoff, and Geiffert in New York, where she was responsible for design and for supervising planting. She opened her own office in 1922. The scope of her practice was extensive, and included private estates, real estate subdivisions, the grounds of industrial plants, recreational developments, and exhibit gardens throughout the United States. Flanders became a member of the American Society of Landscape Architects in 1923, and was elected a Fellow in 1942. In 1932, she was awarded the Medal of Honor in Landscape Architecture of the Architectural League of New York. She relocated her office to Milwaukee in 1943. Flanders lectured extensively to horticultural and botanical societies, schools, and garden and women's clubs, and also appeared often on radio talk shows. She wrote extensively for numerous publications, including , , and .